Attaching/detaching measures and app.reqs not possible (assignment lock)

What causes this issue?

You tried to attach or detach a measure or appropriation request
to/from a program position of investment program &V1&/&V2&. However,
this is no longer possible, since an assignment lock is active in the
master data of investment program &V1&/&V2&.

System Response

The function cannot be performed.

How to fix this error?

If you only want to <ZH>change</> the assignment of a measure or
appropriation request within investment program &V1&/&V2&, you can do
so using the 'Reassign Measures/Appropriation Requests' function
(transaction IM05).
In all other cases, you have to request the removal of the assignment
lock on investment program &V1&/&V2&, if necessary.

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.