SAP Message V2026 - Order & has not been assigned to a sales document

Diagnosis
The system could not determine an SD document item in order &v1&

System Response
The system does not allow you to bill order &v1& and terminatesprocessing.

Procedure
For CO internal orders
Assign a sales document item to the CO internal order. Maintain thisassignment in the master record of the order.
For maintenance orders
You cannot bill a maintenance order. Use a service order instead.