There are various errors that get returned that mention a requirement for a UPC, MPN, Brand or GTIN. These errors can appear in the Channels Errors report in SureDone (navigate to Errors in the left sidebar, then select the eBay tab). An example warning message looks like:
ebayid Main: Warning For multiple-variation listings, GTIN values are specified at the variation level.
Whenever an error of this nature appears — whether in the Channels Errors report or as an error message on your product page — the first step is to make sure you have those fields filled out on 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. Refer to eBay's current Seller Center documentation on product identifiers and item specifics for a list of categories, sub-categories, and required fields.