Date conflicts exist. Check dates

What causes this issue?

The system has discovered scheduling conflicts either when you saved
your data or when you executed the function <PF>Check dates</>.
If you use top-down planning, you cannot enter dates at the
subordinate level that exceed those at the superior level of the WBS.

System Response

INCLUDE 'NA_ALLG_NOFUNKTION' OBJECT DOKU ID TX

How to fix this error?

Adjust the dates at each level using the menu option Edit, by either:
Copying the dates of the superior WBS element to the subordinate WBS
elements or
Copying the dates of the subordinate WBS elements to the superior WBS
element.

Error message extract from SAP system. Copyright SAP SE.