This error means that the required value for UPC was not specified for Products you wanted to list/revise/relist.
Now, it is officially required to provide Product Identifiers for Simple and Variational Products in certain eBay Categories.
Please read more information about in the article.
So, to solve your issue you should go to the Description Policy you are using for that Item and set necessary settings in the eBay Catalog Identifier block - select the product attribute which contains UPC data. Please 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”. Be attentive, as the value has to be exactly the same as provided in the example without any changes. Otherwise, eBay will return an error again.