You cannot install an OTA update on rooted device because SuperSU changes some statement in install-recovery.sh to run itself on boot.
To install OTA you need to install factory image available at https://developers.google.com/android/nexus/images?hl=en
If you dont want to loose your apps and data. Simply edit the script flash-all.bat and remove
-w
From
fastboot -w update image-hammerhead-*.zip
Installation script file name will differ according to your OS.
You will loose root access and TWRP after going through above method as there's no other way to install OTA because all system files should be intact for successful installation of OTA.