Purchase order & not yet released

What causes this issue?

Purchasing document &V1& is subject to a <GL:release_strategy>release
strategy</>. You can only maintain the accounts after Purchasing has
released the document.
The system does not list the items in the purchasing document.

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?

Contact the Purchasing department and ask them to release the document.
You may wish to create a new list of order items and deselect this
purchasing document using the single value selection/deselection
function.
Note: This message only shows you the first purchasing document that is
not released. It could be the case that further purchasing documents
that meet your selection are not released.

Error message extract from SAP system. Copyright SAP SE.