Check the acc. assignment(s), they have been deleted in the pur. document

What causes this issue?

You have held or parked an invoice for a planned account assignment of
a purchase order.
You then deleted this planned account assignment in the purchase order.
When you recall the parked or held document for further processing, the
account assignment is kept in the invoice document as an unplanned
account assignment.

System Response

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

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.