No dates for WBS or project definition exist

What causes this issue?

There must be a time frame in order for you to enter activities for a
WBS element.
It can come from the:
WBS element
superior WBS element in the hierarchy
project definition

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

Maintain the dates in the WBS element or in the superior WBS element.

Error message extract from SAP system. Copyright SAP SE.