Why can't I apply a receipt amount to a closed debit item?

The transaction type of the debit item to which you're applying the receipt is used to validate the application amount. If the transaction type allows overapplication, then you can apply a receipt to a closed debit item.

If the transaction type allows natural application only, then you can't enter an amount that would reverse the sign of the debit item. In this case, you can only enter an amount that brings the balance due closer to zero.