You export your products and you get an error message like this one:

Product/Combination ID: 1/2 - Missing reference

It is a product identifier and its combination identifier in the format 1/2, 1 is the product ID, and 2 the combination one.

This message indicates that the product does not have the “Reference” field filled in, for example:

 

The product reference is missing. Refer to how to Validate your catalog structure.

You can then find a product displayed in this list of identifiers, thanks to its Product ID by searching in your catalog, for example: