I hear you and understand how important it is to be able to accept payments through Merchant Services, @lin5.
I'd suggest getting in touch with our Payments support staff. This way, they can check the cause of this error you're getting.
They can also provide other troubleshooting steps that can help fix the issue.
For your future reference, you can check this article to help customers if they can't pay for an invoice you send them in QuickBooks: What to do if customers can't pay their invoices online.
Leave a comment below if you have any other questions or concerns concerning payment processing. I'll be there to help in any way I can.
