The site is this: https://www.biocanapa.abruzzo.it/car...anapa-p-4.html
in the FAq it says:
SKU / MPN / GTIN

This section describes adding custom fields to your product table.

There is a companion Plugin available to both install custom fields and add the corresponding fields in the admin Product Edit page:

https://github.com/torvista/Zen_Cart...Product_Fields

sku: is populated by products_model. This is the code used by your shop, which is probably unique to your shop.

mpn: is the original manufacturers part number. It is unlikely that you are using that as your shop sku, so you will need to add this column to your products table and populate it.

ALTER TABLE products ADD products_mpn VARCHAR (32) NOT NULL DEFAULT '';

gtin: international identification number that depends on the products you sell: UPC / GTIN-12 / EAN / JAN / ISBN / ITF-14. You'll need to deal with this similarly to mpn.

I used ean and added a products_ean field to the products table:

ALTER TABLE products ADD products_ean VARCHAR (13) NOT NULL DEFAULT '';

In the code, you will need to modify the code to use the column names you have created: the necessary sections for modification are marked CUSTOM CODING.

By default, they are left unpopulated so Google Rich Results Tool will remind you they are missing.

But which file needs to be changed?