Can I use Apple Pay on a series 0 with an iPhone 12?

No.

Apple Pay is compatible with these devices - Apple Support

Apple_Marcin wrote:

Apple Watch (1st generation) is actually Series 0.

Pls correct me if I'm wrong.

Consider yourself corrected.

Note this from the Apple Watch Tech Specs. It shows the difference between the two.

Dear All,

It seems that it was some bug in iOS 14.5.1 and it was causing this problem. I just updated my IPhone SE2020 to new iOS version 14.6 and it fixed the whole problem with adding cards to my Apple Watch Series 0 (first generation, watchOS 4.3.2). Everything works just fine now. Pls update the software on your Iphone and try to add your debit/credit card(s) again.

According to that link it is still supported:

"Apple Watch Series 1 and 2 and later

Apple Watch (1st generation)"

Apple Watch (1st generation) is actually Series 0.

Pls correct me if I'm wrong.