No Command Error on Nexus 5X, even in recovery mode
Download the latest TWRP image.
Download Android SDK platform tools and extract the zip file.
Move the TWRP image inside platform-tools folder.
Boot your phone into bootloader (keep holding volume up + volume down + power until you see the bootloader screen).
Connect your phone to PC / laptop via USB cable.
Navigate to platorm-tools folder which was extracted on PC / laptop.
Hold shift and right click inside empty space in the folder and click Open command windows here.
Type fastboot devices and press enter. You should see your device being listed.
Type fastboot flash recovery twrp-3.1.1-0-bullhead.img and press enter. It should finish flashing in a few seconds.
Press volume down two times on your phone to select Recovery.
Press power button and it should boot to TWRP recovery.