Work center & in plant & is still being used

What causes this issue?

You are attempting to delete a work center or mark a work center for
deletion. However, this work center is still being used in a task list
or a work order.

How to fix this error?

You have the following two choices:
You can delete the work center, once it is no longer being used or
replace this work center with another. You can then delete the replaced
work center.

Error message extract from SAP system. Copyright SAP SE.