When you process a payment towards a customers account and that payment is saved as Account Credit, there is no trail once that account credit is created as to where that credit is used.

Example:

This account has two payments that were processed as account credit. Both account credits have been applied to the invoice (or even to a different invoice) however there's no indication which credit is which.

If you then revert the payment, the system provides a warning indicating that the payment is account credit and the system does not know where you applied the credit. The pop up is providing instructions that you need to move the account credit off of whichever invoice you applied it to and back to account credit first.

If you revert the payment without moving to account credit first, the account credit section will become negative -

To resolve this:

Locate the invoice that the original account credit was applied towards. Select the 3 dots on the right of the invoice and select MOVE PAYMENT TO ACCOUNT CREDIT.