Nexus 9 issues

Purchased a Nexus 9 with android 7. When i root the device and install nethunter the device is in a bootloop. When downgrade to android 6.0.1 (volantis-mob30w-factory-c2c750bc) and then try to either root or install nethunter I get an decryption password in TWRP.

So on android 7 a bootloop and on android 6 a decryption password. Dont know where the encryption is coming from and why this only is on android 6.

Anyone got an idea which exact version of android to use and how to install nethunter?