eBay puts out seller updates every quarter and they have been warning that certain product identifiers, such as UPC, are required when listing in certain categories. Now they prevent you from posting without them in certain categories and also stop revisions of items that already existed if the UPC or other product identifiers are not entered. That's up to eBay. More information on that can be found here - https://pages.ebay.com/sellerinformation/news/springupdate2015/product-identifiers.html
Sku Fetch scrapes the UPC if it is found on the supplier's product page. If it is not, then there's no way for Sku Fetch to obtain it. You can try using a site like https://upcitemdb.com to find the valid UPC for the products, you can purchase your own elsewhere to use, or you can purchase them in your Add Ons Settings and have Sku Fetch automatically insert them into your listings either when no other UPC is found or for all listings.