Purchase order & has no items

What causes this issue?

The system has not found any order items in the
<GL:purchase_order>purchase order</> &V1&.

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?

INCLUDE NA_EINGABE_PRUEFEN
INCLUDE MB_NA_BESTELLUNG_SUCHEN

Error message extract from SAP system. Copyright SAP SE.