Computed Field not showing on Line Item

Hi every body,

I am new in Drupal. I am using Drupal Commerce module to make an e-store. So I have a product name "Licence 1" it has an attribute (field) named "language pack price". in the other side I created a Line Item Type to add a field of language pack quantity to let customer put how many language packs he likes to add with his licence. It works like a charm. But now I would like to show the price of the language pack just after the field of the quantity so basically it is about showing a product field in the item line type. I just found the computed field module I thought it will help me to add a field in the "item line type" that contains a dynamic value got from the $entity variable. When testing it I just put :

$entity_field[0]['value'] = "test";

in the Computed Code (PHP) field
and left

$display_output = $entity_field_item['value'];

in the Display Code (PHP) field

then I checked "Include this field on Add to Cart forms for line items of this type."

but in the add to card page nothing has changed.