Let me help sort this out for you, @lisa43.
You can let your client use another browser to check if this error is browser-related. If he keeps getting the Try back later prompt after a few attempts, it's possible that this is a processing limit.
Let me refer you to this article for more details: Online payments and processing limits.
I'd also recommend reaching out to our Merchant Services Team so they can assist you further using their tools.
If I can be of additional help with any other QuickBooks topic, you can always find me here. Have a good one.