HU & is not a planned HU. You cannot make changes

What causes this issue?

You are currently in a transaction is which you can only change planned
HUs. The HU you want to change is not a planned HU.

System Response

The change is not permitted.

How to fix this error?

Switch to a transaction where you can change the HU.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.