Planning level &1 is not maintained

What causes this issue?

The planning level belonging to the planning type is not maintained in
the system.

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?

Correct your entries or maintain the planning level.
<DS:TRAN.OT12>Execute Function</>

Error message extract from SAP system. Copyright SAP SE.