You export your products and you get an error message like this one:
EAN code missing for the product XXX-YYY(1/2)
For example:
The product has no EAN code, supplier reference and/or brand (manufacturer), therefore it cannot be exported. Follow the instructions on the page « Validate your catalog structure »
This is also valid for the combinations, the combinations must all have a unique EAN field filled in. Please see the page Validate your catalog structure about this subject.