ADB Device Unauthorized [Android 6.0.1 Nexus 6P]
You can reset the list of authorized devices by doing:


  
  Developer Options
  Under Debug Group select "Revoke USB Debug Authorization" (I'm translating from italian but should be something similar)
  Unplug your phone and go to your launcher
  Plug the cable to pc
  Check when you connect the cable that Data transfer is enabled (from the notification that appears)
  


Hope this can fix the issue