ADB Device Unauthorized [Android 6.0.1 Nexus 6P] I've literally tried everything it seems like. There was an another unanswered post on here asking something similar but obviously it didn't help since no one had answered.
RSA Fingerprint Dialog is not displaying - regardless of what I do
My phone used to display the RSA fingerprint dialog after selecting USB Debugging under developers settings. Now it doesn't. I'm very frustrated as I've only purchased this phone for the sole purpose of installing Kali nethunter on it.
I had the phone rooted before and installed the wrong rom for kali nethunter so I flashed the stock ROM back on to it and all is well and working except USB Debugging. I'm about to throw my phone away at this point... I've tried everything it seems:
Past, Frustrating Attempts

Try a different cable
Switch phone to PTP and MTP mode
Switch USB debugging on and off
Install and uninstall universal driver and google drivers
try different usb port
ADB kill-server / ADB start-server (Device still unauthorized under ADB Devices)
Install SDK manager and update ADB / Fastboot
Try a differnet OS (Windows 7, Windows 8.1, Windows 10, Kali Linux)
Factory Reset
Delete RSA Keys under 'C:.android'

All of these did not work.
My phone as it sits is unrooted (because I can't put it in USB Debugging mode) 6.0.1 marshmallow and TWRP installed as custom recovery.
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