When you are uploading your paperback cover to Amazon KDP you have two options for your barcode:
1. You use the barcode that they provide. Leave the bottom right corner of the back cover empty so the barcode can fit. I strongly suggest using their templates. You can access those here: https://kdp.amazon.com/cover-calculator
2. You can provide your own barcode. It should already be on the cover file at the back cover’s bottom. You need to check “provided my own barcode” when uploading.
Best of luck with your book!