New path creation through hook menu problem
You form function callback is incorrect you need to return a valid form, try this code:

function displayproducts_show_form($form, &amp;$form_state){
  $form['item1'] = array(
   '#markup' =&gt; "hello world",
  );
  // @TODO: include more form items.
  return $form;
}


Documentation: FAPI

Make sure that this function is in your uc_cart.admin.inc file as you indicate in the hook menu declaration: 'file' =&gt; 'uc_cart.admin.inc', if it is in your main module file (.module) remove this line and clear the caches again.