Thanks for the question! Let's see if we can get you paid. It may be that something in your customer's cache on their web browser is causing this issue. I would suggest that your customer try some basic web browser troubleshooting - clearing cache and cookies, trying a different browser, etc.
If they still get that message, it may be an issue with the card they are trying to use. I have seen that message when some information they are entering doesn't match what the card-issuing bank has on file. They may need to use a different card than the one they are using when they get that error.
If they still run into trouble, I would reach out to the Payments team directly so they can check the attempts and see if there are any errors on the back end.