Thanks for dropping by the Community space, @jill31.

Recently, there was an investigation about this error prompt when paying invoices. The case is now closed and resolved.

The pop up is a declined response message. I recommend asking your customer to try a different payment method. 

If they keep getting the same result, please have them perform some basic troubleshooting steps. Sometimes cache stored up in the system can cause issues like this, and using a private (incognito) browser usually fixes the problem.

Here are the keyboard shortcuts:

• Command + Shift + N (Safari)
• Ctrl+ Shift + N (Google Chrome)
• Ctrl + Shift + P (Mozilla Firefox)

Once done, they can try paying the invoice again. If it works, have them go back to their default browser and clear cache. Here's an article for the details: Delete browser cache and temporary Internet files.

Otherwise, switch to a different browser like Firefox, Internet Explorer, Google Chrome, or Safari. The one you are currently on may be having a temporary issue with QuickBooks, and using a new browser in the meantime will allow you to get back to work.

Know that I'm here anytime you need help, so stay in touch. Have a great day!