I'm here to help you get past the error that you received when running credit card in QuickBooks Online so you can get back on track.
May I know what is the specific error message did you received while running your customers credit card? It'd be best if you can provide some more details about this concern so I can come up with a correct resolution.
For now, you can try performing some steps in these articles about fixing credit card errors when processing transactions in QuickBooks Online:
Please get back to me with additional information on your concern so we can get this taken care of. You may also reach out to our Merchant Services Department. They can pull up your account securely and help determine the reason for the error that you're getting.
I'll keep an eye for your reply and looking forward to further assisting you. Wishing you and your business continued success.