Release with code & not possible

What causes this issue?

You wish to release the entry sheet using a release code that either
does not exist or is not defined in the release strategy at this point.

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?

Check your data and make any necessary changes.
Information on the release strategy is available in the service entry
sheet via the menu path "Goto -> Display release strategy".

Error message extract from SAP system. Copyright SAP SE.