You cannot enter an amount for this account assignment item

What causes this issue?

You are entering an invoice and want to post additional quantities to
the selected account assignment item. This is not possible, as the
corresponding account assignment item in the purchase order has a
<DS:DE.AA_FINAL_IND>final account assignment indicator</>. The
<DS:DE.AA_FINAL_QTY>final account assignment quantity</> and the invoice
quantities already posted to this account assignment do not allow for
any additional quantity postings to this account assignment item.

System Response

The system deselects the account assignment item.

How to fix this error?

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.