SAP Note 13290 - Not possible to set delete flag (purchase order)

Symptom:

Program : SAPMA02S
Error-ID: AA901 AA903
When you try to flag an asset for deletion (AS06) or to lock an asset for inventory posting, error message AA901 is issued: "Deleting not possible. Open purchase orders exist" or AA903 "Locking not possible. Open purchase orders exist."
However, the purchase order that refers to the asset is no longer open, or has already been archived and thus no longer exists in the R/3 System.

Cause and prerequisites

An asset must remain in the system as an account assignment object for the purchase order as long as the purchase order itself still exists in the system. This guarantees that a goods receipt, for example, can be cancelled for the purchase order at any time.
However, if the purchase order no longer exists, the matchcode for
it can still exist.

Solution
First, the purchase orders for an asset must archived before the
asset can be flagged for deletion or locking. Because purchase orders
are goods movements, a faster reorganization cycle is normally
guaranteed for purchase orders than for assets.
The error message will be changed to remove the confusing
adjective "open" in future.

In any case, you must archive the purchase order(s) before
you reconstruct the matchcodes.

Key word: Delete flag