Start date earlier than start date & in superior WBS element

What causes this issue?

The start date which you entered for the WBS element is earlier than
start date &V1& for the superior WBS element.

System Response

INCLUDE 'NA_ALLG_VERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

If you use top-down planning and shift dates, the dates of the
subordinate WBS elements should lie within the time frame of the
superior WBS element.
The system checks the dates when you save. If there are scheduling
conflicts, you must adjust the dates accordingly.

Error message extract from SAP system. Copyright SAP SE.