I have used Galaxy watches for the last 4 years. My previous watch was a Watch4. I used the NFC feature to pay all my shopping using 2 credit cards. I have just switched to Watch6 and only one of the 2 cards working on my Watch4 will activate on my Watch6. The Watch4 uses Samsung Pay. The 6 uses Samsung Wallet. I had an online chat with Samsung and they said it was a problem with the bank. How come it all works fine on my Watch4? I think incorporating Samsung Pay into Wallet has introduced a bug.
Do you have Samsung Wallet in your phone? Do you have the same issue adding your card there?
It can be an issue with your bank support for the new Samsung Wallet and not an issue on the watch itself.
For more information on how to back up your device: https://www.samsung.com/au/support/mobile-devices/back-up-samsung-device/
To book an appointment prior to heading in-store or to drop off your device via Australia Post: https://www.samsung.com/au/support/your-service/main
To find your nearest store: https://www.samsung.com/au/samsungstore/