There are three versions of the listing form on a desktop - the quick listing tool, the old (full) listing form, and the new (full) listing form. I can't provide a screenshot of the quick listing tool or say where the UPC field is located as I don't have that on any of my accounts, but the following screenshots are from the old and new forms (if your listing form doesn't look like any of these, you likely have the quick listing tool, which you can usually switch from via a link that says 'Switch to advanced listing tool'. If you do not see that link, submit a listing - even if you have to pick 'used' to get it done - and then go in to revise the live listing, as you will then usually get the link to switch).

UPC field location in old listing form:

And UPC field location in the new listing form: