Adding UPC on listing

Hi

Have recently started having a problem when listing items as Brand New or New-Never Used. When I go to list the item, I get the red error message saying that the UPC is missing. However, there is nowhere that I can see on the listing page where i can add this information. If I change the status to "Used", the message disappears and I can list it. Obviously do not want to list something as used when it is new, Can anyone help me with this?