You export your products and you get an error message like this one:
EAN code missing for the products, References: [demo_1, demo_2]
It is a list of product references, you can find the product by pasting the reference in the search bar of Prestashop.
This list shows all the products that have an unfilled “EAN” field, par example:
This is also valid for the combinations, the combinations should all have a unique EAN field filled in. Please see the page Validate your catalog structure about this subject.