There are 2 possible reasons for getting this error on a product:
An originally single variant product now being sent as a multi-variant product - eBay will not allow this so the product needs to be completely ended, deleted and re-listed
The variation data for the product from the eCommerce store is not good particularly good e.g. different attribute names in different variants.
So, it would have originally listed fine but now is not getting updated due to failing the validation rules.