My client is trying to pay me. She gets an error message on the top of the invoice that states..."error-you can try to again or come back later."

Hi lgm2. Recently there was an investigation about a similar issue. This investigation however has already been closed which leads me to believe this is a simple error with your client's web browser. To test this out, have your client try using a private browser first. 

Here’s how to do the private browsing:

• Safari: Control + Option + P
• Google Chrome: Press Ctrl + Shift + N
• Firefox or Internet Explorer: Ctrl + Shift + P

If this works, you can clear your browser's cache to refresh it. Click this link for the guide: Delete or disable cache and temporary internet files in your web browser.

If there's anything else I can do for you, I'm always here to help. Have a great day.