Commerce 2 | Quantity Dropdown with Default Values -> Error
I found the solution to my issue:  

 function mymodule_form_alter(&amp;$form, $form_state, $form_id) {

 if (strpos($form_id, "commerce_order_item_add_to_cart_form") !== false) {
$form['quantity']['widget'][0]['value']['#default_value'] = 1;
 $form['quantity']['widget'][0]['value']['#step'] = 0.5;
   $form['quantity']['widget'][0]['value']['#min'] = 1;
    $form['quantity']['widget'][0]['value']['#max'] = 3;
    $form['quantity']['widget'][0]['value']['#type'] = 'select';       
 $form['quantity']['widget'][0]['value']['#options'] = array(
 '1' =&gt; t('1 hour'),
'1.5' =&gt; t('1,5 hour'),   
  '2' =&gt; t('2 hours'),
   '2.5' =&gt; t('2,5 hours'),
   '3' =&gt; t('3 hours') );
    }}