If a Customer has an outstanding Credit Memo, at least one of the Customer's ROA payments has not been fully applied.
If possible, fully apply all Credit Memos before entering new ROA Payments.
There are two actions that produce a Credit Memo. The Customer made an ROA Payment that was greater than the account balance, or a Clerk entered a Credit Memo in the Sales Manager using the Credit Memo function ([Shift]+[F8]).
See:
Accounts Receivable Contents