I have gone through this loop multiple times.....same result.

-Nexus 9 - running 6.01

-Unlock bootloader - successful

-Root device - TWRP installs fine and SuperSU and Busybox setup.

-I try to install the nethunter image from the Nexus Root Toolkit - seems to install correctly, but then when it reboots.......Google screen (with just Google on it - not the rotating graphics) just keeps rebooting.....back to the Google logo......over and over.

If I try to enter Recovery (the TWRP) - it tells me command not found.