Plan version & is not defined

What causes this issue?

You have either forgotten to enter a target or reference version, or
the target or reference version you have entered does not exist or does
not exist in this fiscal year.

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?

In the first case, you can enter the version you have forgotten.
In the second case, you should recheck the entry for the controlling
area, fiscal year and the reference or target version.

Error message extract from SAP system. Copyright SAP SE.