Set default value for Enity Reference using View and Entity Reference View Widget Checkbox
Did you try casting the value to a string?  I had to do that when I was setting default values for a user reference field type.  It wouldn't work otherwise. Something like

 $form['line_item_fields']['field_pesonal_data']['und'][0]['field_cover']['#default_value']=(string)6;


Also, I noticed that you had a spelling error.  You had 'field_pesonal_data' It should probably be 'field_personal_data'

Also, I was a little confused by your use of terminology.  You described that the user is configuring the entity reference field on the product.  Did you mean configuring the entity on the line item?  I just wanted to clarify, because if you are modifying the form that governs the adding of products (or product variations in Commerce Kickstart 2.0), you need to use a different form alter call because the product variation creation form uses the inline_entity_form module to manage the specific product variations.

That call is:

function hook_inline_entity_form_entity_form_alter(&amp;$entity_form, &amp;$form_state)


Look at the inline_entity_form.api in the contrib/inline_entity_form folder in Commerce Kickstart 2.0

for modifying a user reference on a product, I did the following:

function bh_vendor_inline_entity_form_entity_form_alter(&amp;$entity_form, &amp;$form_state) {
  global $user;
  if ($entity_form['#entity_type'] == 'commerce_product') {
    $entity_form['vendor_user_reference']['und']['#default_value'] = (string)$user-&gt;uid;
  }
}


My widget I used was a option list.
You have to have the ['und'] or it won't work either.