It's possible that there is an issue with your QuickBooks payment account. This can be the reason why you're unable to process your payment on your card reader.
I'd recommend contacting our QuickBooks Payments Support team so they can take a look at your account. This way, we can identify the cause of the issue and address it. You can reach out to them by clicking this link: Contact Payments Support.
In addition, you can sign in to the Merchant Service Center from a web browser. This is where you can manage your payment account in the Merchant Service Center and check your status.
Let me know how else I can help you with QuickBooks by adding a comment below. I'm more than happy to lend a helping hand. Keep safe!