Online and Offline scenes should be registered in Build settings beforre you can set them to appropriate slots in NetworkManager, if scene is not registered and you try to set it to the slot - it should give warning - the problem is that right now it gives 2 almost the same warnings!
When user tries to set

How we can reproduce it using the example you attached
1 Create new project
2 Create new scene and save it
3 Create empty object and add NetworkManager component to it
4 Try to drag scene asset from Project view to Online or Offline scene slot in NetworkManager in Inspector

Actual result:
2 almost the same warnings appear:
- Scene [OfflineScene] cannot be used with networking. Add this scene to the 'Scenes in the Build' in build settings.
- The scene OfflineScene cannot be used. To use this scene add it to the build settings for the project

Expected result:
one warning is enough :)