First go in TWRP and wipe cache and dalvik cache. Then try again. This fixed so many things for me already.
You do not need to root with superSU (or Magisk) if you plan on installing a new ROM. That would wipe it anyway.
Try to follow the instructions of the official Nethunter thread on XDA:
https://forum.xda-developers.com/nexus-6p/development/rom-kali-nethunter-huawei-nexus-6p-t4080807
You should flash the newest android version from google via fastboot beforehand though:
https://developers.google.com/android/images