There are 6 simple steps for rooting Android 7.1.1 for nexus 6:
1 adb reboot bootloader
2 fastboot oem unlock [YES]
3 fastboot flash recovery twrp-3.2.1-0-shamu.img
https://dl.twrp.me/shamu/twrp-3.2.1-0-shamu.img.html
4 go to recovery
5 adb push .\SuperSU-v2.82-201705271822.zip /sdcard/
https://forum.xda-developers.com/apps/supersu/stable-2016-09-01supersu-v2-78-release-t3452703 https://s3-us-west-2.amazonaws.com/supersu/download/zip/SuperSU-v2.82-201705271822.zip
6 in recovery install SuperSUfrom sdcard
7 done.