NAV - Cannot post because you did not specify which entry to apply. You must specify an entry in the Applies-to-ID field for one or more open entries.

This occurs when attempting to apply customer or vendor payments to invoices.

This is happening because you did not choose "Payment" or "Credit Memo" as the document type when entering the transaction, which would have prompted you to enter the transaction as a negative.
Most likely, your transaction has gone in as a positive with no document type, causing the system to treat it as another invoice.

You cannot apply an invoice to an invoice.

• Solution:
• Reverse the payment transaction
• Redo the transaction with the correct document type
• enter the transaction amount as negative