Could not add card or "apple pay is temporarily unavailable"

After upgrading my phone and trying to verify my credits cards, one of my cards keeps giving this error when I try to verify the CVV number.

So I deleted the card completely from Apple Wallet and proceeded to add it again. When I clicked on Add Credit or Debit card I noticed that apple wallet pre populated my credit card (the one that I deleted) and apparently it had cached an old/expired expiration date which was causing the trouble. I even tried using the Add a different card option and after entering my card number manually it was still caching my old/expired expiration date and refused to add the card with the error in the subject line.

After talking to Apple support (after a 30 minute wait, even my local utility company now has an automated call back option), they confirmed that Apple's servers cache the old data for upto 24 hours.

The final solution I found was:

1. Wait for 24 hours for apple server to mark the data stale
2. Open the Settings -> Wallet option
3. Add the credit card (it will still pre populate the number)
4. Click on Add a different card
5. Click on Manually enter the card number and enter the Name/Card number
6. Click Next and it will show you the old cached expiration date, delete it and enter the new expiration date (this was the problem, for the first 24 hours it won't overwrite this)
7. Enter the CVV number and add the card

Hope this helps other who may be facing the same issue.

Some other tips which may help also:

1. Reboot the phone
2. Check that Cellular data is enabled for Wallet

BTW this line for Apple - this forum sucks (like your call waiting from the 60's), under the devices there's no option to select iPhone 12, 12 Pro or 12 Pro Max (it's still only showing iPhone 11).