The purchase order has not been released

What causes this issue?

You have chosen a purchase order that is subject to a release
(approval) strategy.

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?

Before you can perform the function, the purchase order must be
released (approved).
You can display the release strategy, the release status, and the
release prerequisites applicable to the PO. To do so, choose "Header ->
Release strategy" from the purchase order display.

Error message extract from SAP system. Copyright SAP SE.