The work center is already being used, update task lists

What causes this issue?

You want to change the standard value key for a work center. However,
this work center is already used in task lists, which means you may
have to correct the standard values.
When you remove the reference indicator for a <GL:work_center>work
center</> used in a routing, the <GL:default_value>default values</>
from the work center are not copied to the routing. You must use the
mass replacement function or replace these values manually in the
routing.

How to fix this error?

Select all task lists where the work center is used by using a
where-used list (menu options <LS>Reporting -> Where-used -> Work
center</>).
On the operation detail screen, correct and save the standard values.
RESET N1

Error message extract from SAP system. Copyright SAP SE.