SOLUTION!
In the file: modules/products/Ajax.php
Add this variable on line 48:
‘default_item_tax_rate’ => $ default_item_tax_rate
Being that way:
$data = array (
‘products’ => $products,
‘families’ => $families,
‘filter_product’ => $filter_product,
‘filter_family’ => $filter_family,
‘default_item_tax_rate’ => $default_item_tax_rate
);