You cannot change the reference

What causes this issue?

The operation referenced an operation set in the system until now. You
cannot use Direct Input to overwrite the operation with an operation
that does not have a reference.

How to fix this error?

If you want to remove the reference to the operation set, select one of
the following options:
Delete the operation using Direct Input and create a new operation.
Unlock the reference to the operation set in the system before you start
Direct Input. The you can overwrite the operation without any problems.

Error message extract from SAP system. Copyright SAP SE.