You cannot add new items in this purchasing document for Advanced Returns

What causes this issue?

This purchasing document (returns purchase order or returns stock
transport order) is a follow-up document for a customer returns process
in Advanced Returns Management.

System Response

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

How to fix this error?

Create a separate returns purchase order or returns stock transport
order for the items you want to add.

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.