How do I see a specific customer's available credit? Is this different from a credit memo?

"How do I see a specific customer's available credit?"

By using the Icon at the top Ribbon of Invoice, you see if there is a Credit Available. You also can run an AR report filtered on Amount <= 0, to see the Negative AR. Please see my attachments.

"Is this different from a credit memo?"

Sort of. A customer can have a negative AR from various conditions: A credit memo that you did not refund; an overpayment that was not refunded. Please see my attachments.

"When I create an invoice for XYZ Customer, the message "This customer has available credits that can be applied to this invoice. Would you like to apply those credits to this invoice now?" keeps appearing."

And your pop up shows a Credit Balance from a date that is squished, but the amount originally was $13,527. You are whittling it down as you use it, and you see this in that Line entry. You used $2,140 of it here, leaving over $11,000 still on hand and to be used.

Does that not make sense to you, for this customer?

"The problem is, is that I don't recall ever creating a credit for this customer. I checked credit memos and I don't have any credit memos for this customer either. "

Did you Receive Payment of over $13,000 and forgot to make an invoice for that sale? Widen that column to see the date from "11/02/20..." and run a Customer Open Balance report, to see what else has been entered.