Object is used in an operation marked for deletion.

What causes this issue?

The object (for example, work center, reference operation set or
production resource/tool) is not used in any task list. However, it is
assigned to operations marked for deletion.

How to fix this error?

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

Reorganize the task list in which you are starting an archiving run
with a deletion program.
Then delete the object.

Error message extract from SAP system. Copyright SAP SE.