I’ve been getting the same error when clients try to pay the invoice online. Everything is on settings and have been assisted by QB technical support. Was told they will contact me after investigating the issue. I was told that on two different occasions and still no fix. I am currently waiting to hear back from technical support, but with the previous support, they will probably close the case after two weeks and not contact me. This seems to be an ongoing issue based on this thread. It’s impossible to process card payments and creates additional time for cash flow due to checks being the only way to process payments. I will have to look for alternatives to QB soon like Xero or Freshbooks if the problem isn’t solved soon.