Enter start and finish date

What causes this issue?

You have only entered one date for the WBS element (either the start or
the finish date) however both dates are required for time scheduling.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

Enter the missing start or finish date for the WBS element.

Error message extract from SAP system. Copyright SAP SE.