Ah - that's your problem then, you can't just make them up. It might be worth a Google search but as far as I am aware there is no way you can just create valid ones yourself, you need to buy them from an official supplier. The biggest supplier is https://www.gs1.org/ but you can buy them off eBay etc. Only word of warning is if you every want to sell on Amazon then they are really fussy about barcodes and often reject ones bought off eBay. GS1 is the safest route but they are expensive.
Other option of course is not to use barcodes at all. You can tell Google in your feed that these products don't have one and Google (and eBay) will let them through.
I'm having the same problem in this thread. I used the barcode generator app and it created 8 digit barcodes.
8 of 80 products were approved ...... I cannot figure out how those are different than any others???
How do you tell Google in your feed that products don't have bardcodes to be able to utilize google shopping?