Most often the issue is a manual entry error and the number hasn't been entered correctly. We would first recommend trying the same card number again.
Then, try hard-exiting/closing the app and re-opening it.
If your clinic is part of our GoFetch network, they also now have the ability to generate a Virtual Card on your behalf at check-out. If you don't have access to your GoFetch app or are facing any issue generating your virtual card, they may assist you.