If you weren't on the beta version of Android Nougat on your Nexus device then you may not have received the official update to Nougat since beta devices were the first to receive the update. In this case you would need to flash the update manually via fastboot. If you flash Nougat with Google's official system images you won't be able to flash TWRP and root since they force encrypt.
Wugfresh the guy behind the Nexus Root Toolkit has put together no force encrypt images for all the Nexus devices. Once these are installed you should be able to easily root your device with a custom recovery. Grab the system image for your device at the link below.
via AFH