Found a similar case here but since it was on a 2 year old thread I decided to make a fresh one. Can't add Curve to Google pay (contactless) on my Smart Watch - #3 by Prodweb
Simply put, I can’t seem to get Google Pay and Curve working on my Watch4 Classic. All I get is a notification saying the card is already added on this device even though no card shows up on the app itself. It’s very odd since I already got the Curve contactless payments working on my regular Watch4 before moving to the Classic.
I know the phone and watch versions of Pay save the cards independently, so I also tried removing the card from my phone and adding it to just the watch. Same issue. “Card is already added”. After doing this a few more times and contacting Google support I now can’t even get the contactless feature working on my phone anymore. The problem seems to be that I’m no longer getting the SMS verificarion code (as was mentioned in the original problem thread I linked).
I hope there is a fix for this.