There are various errors that get returned that mention a requirement for a UPC, MPN, Brand or GTIN. The following is an example of an error that can be seen in your results file for a bulk upload:
The error says “ebayid Main: Warning for multiple-variation listings, GTIN values are specified at the variation level”.
Whenever an error of this nature is returned whether in your results file or an error message that appears on your product page, the first step to take it make sure you have those fields filled out in the Product Page for the item. If those fields are filled in, then most likely you need to find out if that field is an item specific for eBay. The following eBay link lists all different categories as well sub-categories and what fields are required: http://pages.ebay.com/sell/item_specifics/product_identifiers.html