Cause
The screen variants have not been correctly defined in customizing.
Resolution
The flexible detail screen have to be deactivated for the project profile,
Or the Customizing has to be completed for these detail screens.
To define display variant for detail screens of the WBS elements in customizing follow the path
SPRO
> Project Systems
> Structures
> Operating Structures
> Work Breakdown Structures (WBS)
> User Interface Settings
> Layout of WBS Element Detail Screens