Thanks for posting this situation to the Community, Pair1,
Let me get the help you need to ensure you'd able to accept credit card payments.
If I may ask, have you encountered any issues upon accepting credit card payments? This is to ensure that I'm able to provide you with the best resolution since you've already done some troubleshooting steps to resolve the issue.
In the meantime, I'd suggest clearing your app's data for the last time to fix any errors in the system.
To do this task, here's what you'll need to do:
Sign in to your QuickBooks Online app.
Click the Help menu at the top.
Choose Reset App Data.
Once done, close and reopen the app, then try to accept credit card payments again.
For additional reference, I'm attaching some helpful articles that you can visit: