No activities for selected & found

What causes this issue?

You selected WBS elements and want to include activities. The system
could not find any activities for the selected WBS elements.
Possible causes are:
No activities exist for the selected WBS elements.
<ZH>or</>
Activities exist for the selected WBS elements but have been deleted.
Although these activities may still be in the database, you can no
longer display or change them.

Error message extract from SAP system. Copyright SAP SE.