This error means that the required value for UPC was not specified for Products you wanted to list/revise/relist.
eBay requires providing Product Identifiers for Simple and Variational Products in certain categories. To solve your issue you should go to eBay > Configuration > Policies and edit the Description Policy you are using for the Item and set necessary settings in the eBay Catalog Identifier block. Select the product attribute which contains UPC data.
Note: only attributes which are presented in all attribute sets are shown in the dropdown.
In case your product(s) has no UPC value, you can provide it as “Does not apply”.
Read more about Product Identifiers in the article.