Bootloader unlocking veterans probably did the same thing I did when they first got their Nexus 6P. Of course the first thing I wanted to do was get that bootloader unlocked and get TWRP installed. I plugged my 6P into my PC and proceeded to boot the phone into fastboot mode so I could unlock the bootloader. After typing the normal command "fastboot oem unlock" and getting an error I scratched my head. In these situations google is indeed your friend.
What I found within a few minutes of research is that the Nexus 6P requires a new command for unlocking the bootloader. The new command is as folows,
"fastboot flashing unlock"
Other commands include:
fastboot flashing lock
fastboot flashing unlock_critical
fastboot flashing lock_critical
fastboot flashing get_unlock_ability
I am not sure why the change occurred, but this is generally the same easy process you just have to remember to include the "flashing"!