I had this issue as well when deleting my cards and adding them back in, due to weird issues with payments declined at terminals. Played with settings and found this solution. Go to settings and tap your name/Apple ID bar located at the very top level. Under payment & shipping, tap your payment card and enter the expiry date and/or verification number located on your credit card if it is blank. You can also do this in the App Store or the Apple Store app wherever “payment information”is located. After doing this, and the verification is complete, jump over to the wallet app and try adding your card again. Note that this worked for me only because it was the same credit card I use for my Apple ID. It also doesn’t hurt to call your issuing bank in advance as they can sometimes have a block on your card. Seems to be a bug with the ”handshake” system between Apple and your bank. Following this step finally allowed me to add my main Pay card back in. I really hope this helps as I know how frustrating things like this are.
