In NetworkIdentity user can set both Server Only and Local Player Authority flags, but this doesn't make any sense.

So, probably it would be good to automatically disable and turn off "Local Player Authority" checkBox if user sets "Server Only"

2. How we can reproduce it using the example you attached
1 Create new project
2 Create Cube and add NetworkIdentity - you can see there "Local Player Authority" and "Server Only" checkBoxes
3 You can set both these flags - but this doesn't make sense