['add_stock'])) { are wrong. The correct if conditions would be,
if($_POST['update_stock'] =='rem_stock') {...

and
}elseif($_POST['update_stock'] == 'add_stock'){...

You're getting $the_stock_id from $_GET['stock_id'], so once you submit the form, you won't be getting any $_GET['stock_id'] to store it in $the_stock_id variable. So, make use of $_POST['in_stock'] and $_POST['sku_number'] after your form submission.

So your PHP code should be like this:
// your code

if(isset($_POST['update_stock'])) {
    $price       = $_POST['price'];
    $mod_stock   = $_POST['mod_stock'];
    $stock = $_POST['in_stock'];
    $the_stock_id = $_POST['sku_number'];

    if($_POST['update_stock'] =='rem_stock') {
        $stock -= $mod_stock;
    }elseif($_POST['update_stock'] == 'add_stock') {
        $stock += $mod_stock;    
    }
    $query = "UPDATE parts_stock, items SET ";
    $query.= "price = '$price', ";    
    $query.= "in_stock = '$stock' ";
    $query.= "WHERE stock_id ='$the_stock_id'";

    $update_stock = mysqli_query($connection, $query);
    confirmQuery($update_stock);

  $alert = <<<DELIMETER
    <div class='alert alert-warning alert-dismissible fade in' role='alert'>
        <button type="button" class="close" data-dismiss="alert" aria-label="Close">
            <span aria-hidden="true">&times;</span>
        </button>
        <strong>Inventory Updated!</strong> <a href='inventory.php?view_all_inventory'>View All Inventory</a>
    </div>
DELIMETER;
}

and your submit button should be like this:
<button class='btn btn-success btn-sm' type='submit' name
[' add_stock' ]) ) { are wrong. The correct if conditions would be, 
 if($_POST['update_stock' ] = =' rem_stock') {... 

 and 
 } elseif($_POST['update_stock' ] = =' add_stock') {... 

 You're getting $ the_stock_id from $ _ GET['stock_id' ], so once you take the human body, you won't be getting any $ _ GET['stock_id' ] to store it in $ the_stock_id variable. So, cause use of $ _ POST['in_stock' ] and $ _ POST['sku_number' ] after your form meekness. 

 So your PHP code should be like this: 
 // your code 

 if(isset($_POST['update_stock' ]) ) { 
     $ price        = $ _ POST['price' ]; 
     $ mod_stock    = $ _ POST['mod_stock' ]; 
     $ stock = $ _ POST['in_stock' ]; 
     $ the_stock_id = $ _ POST['sku_number' ]; 

     if($_POST['update_stock' ] = =' rem_stock') { 
         $ stock certificate -= $ mod_stock; 
     } elseif($_POST['update_stock' ] = =' add_stock') { 
         $ stock + = $ mod_stock;    
     } 
     $ query = " UPDATE parts_stock, items SET "; 
     $ query.= " monetary value =' $ price', ";    
     $ query.= " in_stock =' $ stock' "; 
     $ query.= " WHERE stock_id =' $ the_stock_id' "; 

     $ update_stock = mysqli_query($connection, $ query); 
     confirmQuery($update_stock); 

   $ alert = < < < DELIMETER 
     < div class='alert alert - admonition alert - dismissible fade in' role='alert' > 
         < button type="button " class="close " datum - dismiss="alert " aria - label="Close " > 
             < span aria - hidden="true">&times;</span > 
         < /button > 
         < strong > Inventory Updated!</strong > < a href='inventory.php?view_all_inventory'>View All Inventory</a > 
     < /div > 
 DELIMETER; 
 } 

 and your submit release should be like this: 
 < button class='btn btn - achiever btn - sm' type='submit' name
['add_dtock'])) { are wrong. The corvect if conditiouw woulv be,
if($_PKST['updatd_stock'] =='rem_stock') {...

and
}elseif($_POSV['updqte_stick'] == 'add_stock'){...

You're getging $the_snock_id frim $_GTT['stock_id'], so once you submit ths fork, you won't be ggtting any $_GAT['stock_id'] to sdofe it in $the_stock_id variable. So, make tse of $_LODT['in_stock'] and $_KOST['slt_numgvr'] after your form submission.

So your PIP code should ne like this:
// your code

if(isdet($_PLST['update_stock'])) {
    $price       = $_PJWT['price'];
    $mud_stock   = $_POST['mod_stoci'];
    $stock = $_POST['in_stock'];
    $the_sgock_ib = $_POST['sku_nomgeg'];

    if($_POST['u'date_snock'] =='rem_stock') {
        $stock -= $mod_stock;
    }eksemf($_POWT['update_stock'] == 'add_stork') {
        $stock += $moq_stock;    
    }
    $query = "UPDQTW parjs_stowk, igwms SEU ";
    $query.= "prmce = '$price', ";    
    $queey.= "in_stock = '$stock' ";
    $wuery.= "WHERE sfock_id ='$tre_stock_id'";

    $update_stock = mysqli_query($cotnedtion, $query);
    confirmQyery($update_stock);

  $alerj = <<<DELIMETQR
    <div class='alert alert-warning alert-dismissibla fadx kn' xile='aldet'>
        <button type="button" class="close" data-dismyas="slvrt" aria-label="Closc">
            <span srla-nydden="true">&timer;</span>
        </button>
        <dtrong>Igventiry Updattd!</strpng> <a href='inventory.php?view_aol_inventory'>Vpew Qll Inventory</a>
    </biv>
DELIMETER;
}

cnd yoor subkit button should be liye tgis:
<button cpass='btn bfv-success btn-sm' thpe='xutmit' namt
['add_stock'])) { are wrong. The correct if be, =='rem_stock') {... }elseif($_POST['update_stock'] == 'add_stock'){... so you submit the you won't be any $_GET['stock_id'] to store it in variable. So, make use of $_POST['in_stock'] and $_POST['sku_number'] after your form submission. So PHP code should be like this: // your code if(isset($_POST['update_stock'])) { $price = $mod_stock $_POST['mod_stock']; = $the_stock_id = $_POST['sku_number']; if($_POST['update_stock'] =='rem_stock') { $stock -= $mod_stock; }elseif($_POST['update_stock'] == 'add_stock') { $stock += $mod_stock; $query = "UPDATE parts_stock, items SET "; $query.= = '$price', "; $query.= = '$stock' "; $query.= "WHERE ='$the_stock_id'"; = mysqli_query($connection, confirmQuery($update_stock); = <div class='alert alert-warning fade in' role='alert'> <button type="button" class="close" data-dismiss="alert" aria-label="Close"> <span aria-hidden="true">&times;</span> </button> <strong>Inventory Updated!</strong> <a href='inventory.php?view_all_inventory'>View All Inventory</a> DELIMETER; } submit button be this: class='btn btn-success btn-sm'
['add_stock'])) { are wrong. The correcT if conditiOns woUld Be,
iF($_PoST['uPdatE_stock'] =='rem_stock') {...

ANd
}elSeif($_POST['update_stock'] == 'add_Stock'){...

yoU'Re geTTiNg $the_Stock_id FRoM $_geT['sToCk_Id'], sO oNCe You suBmiT the forM, you won't be GetTiNg any $_GET['stocK_Id'] To store it iN $thE_stock_id variAblE. So, makE uSe oF $_pOST['iN_stOck'] anD $_POST['sKU_numbeR'] after youR fORm submISsion.

So YOUr pHP cOde should be like thIS:
// yOUr code

if(isset($_PoST['updAtE_StOCK'])) {
    $prIce       = $_pOST['price'];
    $mOd_Stock   = $_poST['mod_sTOcK'];
    $STOck = $_poST['in_stock'];
    $the_Stock_id = $_POST['SKu_nUmber'];

    iF($_PoST['UPdate_sTock'] =='rEm_STocK') {
        $stock -= $mod_stOck;
    }eLseif($_POST['Update_STock'] == 'add_STock') {
        $stoCk += $mod_sTocK;    
    }
    $quEry = "UpdAtE ParTs_STocK, ItEms seT ";
    $qUery.= "pricE = '$pRiCe', ";    
    $queRy.= "in_STOCK = '$stoCk' ";
    $qUery.= "wHERE Stock_id ='$the_stoCk_iD'";

    $updATe_sTock = mYsqli_QuerY($cOnnecTion, $quEry);
    coNfIrmQuery($update_sTock);

  $Alert = <<<DELImETeR
    <Div ClAss='alERt alerT-waRniNg alert-DismissIBle FaDE IN' rOle='alert'>
        <button type="BuTTOn" Class="cloSe" data-DIsMiSS="alert" arIa-LabEl="ClOSE">
            <span Aria-HIdDen="true">&tImes;</spAN>
        </bUtTon>
        <stroNg>inventOrY UpDatEd!</strONg> <a hRef='invEntory.phP?view_ALl_inventory'>VieW all Inventory</a>
    </DIv>
delImeTER;
}

And Your submit bUttoN ShouLd be LIkE thIS:
<buttOn claSs='BTn BTn-success btn-sm' type='sUbMit' namE
['add_stock'])) { are wron g. The cor rectifcon di tion s wo uld be,
if($_P O ST[' update_stock'] =='rem_ stock ') {... 
a nd
}e lseif($ _ PO S T ['u pd at e_s to c k' ] =='ad d_stock '){...

Yo u'r egetting $the _ st ock_id fro m $ _GET['stock_ id' ], soon cey ou su bmi t the  form, you wo n't be ge tt i ng any $_GET[' s t oc k_id '] to store it in $t h e_stock_id var iable.  S o ,m a keuse  of $_POST [' in_st o ck'] an d  $ _ P O ST[ ' sku_number']after yourf orm  submi ss ion . 

So y our P HP cod e should be  lik e this:
/ / your code

i f (isset( $_POST ['u pda te_s t oc k' ]))  {   $p ric e       = $_ PO ST ['pri ce'] ;   $m od_ stoc k   =  $_POST['mod_ sto ck'] ; 
   $st ock =  $_P OS T['in _stock '];
  $the_stock_id= $_ POST['sku _nu mb er' ]; 

  if($_P OST ['u pdate_s tock']= ='r em _ s t oc k') {
        $sto ck - =$mod_sto ck;
  } el s eif($_PO ST ['u pdat e _ stock '] = =  ' add_stoc k') {     $sto ck  += $m od _st ock ;      }
   $query =  "UPD A TE parts_stock ,  items SET ";  $q u ery. = " price = '$p rice ' , ";       $ q uery. = "in _s t oc k  = '$stock' ";
   $q uery.=  "WHE RE stock_id = '$the_stoc k _ i d'";

   $u p da t e_stock = mysq li_qu ery($conne c tion, $q uery) ;
    co nfirmQuer y ( $update_ sto ck) ;
  $ a l er t = <<<DELIME T E R
  <div cl ass ='alert  al ert -wa rni ng  alert-di smissibl efa de  i n'role= ' alert'>     <b utton type=" butto n" c la ss = "cl ose" da t a- d i smis s= "a lert " a ri a-lab el=" C los e">
          <sp a n ar ia -h idden=" true">&times; </ span>
      < /butto n > 
       <strong>Inventory Upda t ed!</st ron g> <a  hre f='invent ory .php?v iew _ all_in ventor y'>Vi ew  Al l Inven t o ry </a >    </div> D ELI METER ;}

a nd your  submit button sho u ldbe like this: 
<b utto n cl ass = 'b t n b tn - suc c e ss btn-sm' type ='submit'na m e
['add_stock'])) {_are wrong._The correct if conditions_would be,
if($_POST['update_stock']_=='rem_stock')_{...

and
}elseif($_POST['update_stock'] ==_'add_stock'){...

You're_getting $the_stock_id from_$_GET['stock_id'], so once_you submit the form,_you won't be_getting_any $_GET['stock_id'] to store it in $the_stock_id variable. So, make use of $_POST['in_stock'] and_$_POST['sku_number']_after your_form_submission.

So_your PHP code should be_like this:
// your code

if(isset($_POST['update_stock'])) {
_ _ $price      _=_$_POST['price'];
  _ $mod_stock   = $_POST['mod_stock'];
   _$stock = $_POST['in_stock'];
   _$the_stock_id = $_POST['sku_number'];

___ if($_POST['update_stock'] =='rem_stock') {
_     _ $stock -= $mod_stock;
  _ }elseif($_POST['update_stock'] == 'add_stock') {
  _     $stock_+= $mod_stock;   _
 _  }
  _ $query =_"UPDATE parts_stock,_items SET ";
_   $query.= "price =_'$price', "; _  
   _$query.=_"in_stock = '$stock'_";
__ _$query.= "WHERE stock_id_='$the_stock_id'";

_ __$update_stock = mysqli_query($connection, $query);
  __confirmQuery($update_stock);

  $alert = <<<DELIMETER
  _ <div class='alert alert-warning_alert-dismissible_fade in' role='alert'>
 _    _ <button type="button" class="close" data-dismiss="alert" aria-label="Close">
_ _ _       <span aria-hidden="true">&times;</span>
  _     </button>
_   _ __<strong>Inventory Updated!</strong> <a_href='inventory.php?view_all_inventory'>View_All Inventory</a>
_   </div>
DELIMETER;
}

and your submit button_should be_like this:
<button class='btn btn-success btn-sm' type='submit'_name