Production resource/tool in use - Base unit of measure cannot be changed.

What causes this issue?

You are trying to change the base unit of measure for
<GL:production_resource/tool>production resources/tools</>. However
because the production resource/tool is used in one or more
<GL:standard_task_list>task lists</> or <GL:work_order>work orders</>,
the base unit of measure cannot be changed anymore.

Error message extract from SAP system. Copyright SAP SE.