You cannot add new items in a replacement purchase order

What causes this issue?

This purchase order for replacement materials is related to a returns
purchase order in
<DS:GLOS.47AD987B634839F4E10000000A421392>Advanced Returns Management.
</>
To ensure data consistency for supplier returns processes in Advanced
Returns Management, you can only create purchase orders for replacement
materials by using the <LS>Receive Replacement Materials from Supplier
</> transaction (MSR_VRM_GR).

System Response

The system does not allow adding new items in this document.

How to fix this error?

If the purchase order for replacement materials has been created
incorrectly, delete the incorrect purchase order items and create a new
purchase order using the <LS>Receive Replacement Materials from Supplier
</> transaction.

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.