We guys can't help you out. We guys are users like you on this user to user only forum. But Apple won't help you out either. Even with the receipt. They would ONLY help the original buyer who locked the watch to their Apple ID with the original purchase receipt, which clearly shows the serial number of the watch. You're not the original buyer and you can't present the original purchase receipt.
Rent a Center didn't give you a good deal as they sold you a useless watch, you simply cannot use. Either get your money back or if they won't refund you, recycle the watch. You'll never be able to use it.
Read this --> https://discussions.apple.com/docs/DOC-250001038
Ps: You can try what Limnos provided above, but to my knowledge, Rent a Center is not an Authorized Reseller of Apple Devices, so I will be beyond surprised if Apple agrees to unlock the watch. If it were me, I'd insist on a refund from Rent a Center and recognize that if a deal is too good to be true, it usually is too good to be true.