No Command Error on Nexus 5X, even in recovery mode I used Wugfresh' root installer before and it worked like a charm. You can't get any OTA's with it though. Since I wanted Android 7, I wanted to unroot, install the OTA and reroot using Wugfresh' client. I already did this before when I wanted to upgrade to Android 6. Then everything went well and I was done in an hour.

This time however, I'm stuck. When it needed to reboot after succesfully installing Android 7.1.1, I got set to the No Command screen. After some googling I found out that you had to go to the bootloader first, and then go to Recovery. I got that far, but now I have no clue on what to do.

Rebooting from there doesn't work (obviously). It seems logical to me that I have to install TWRP, but the zip file is not there anymore to install it. Is there a way to still get this zip file on my phone while in recovery mode, and is this the solution to my problem? I also found the format option in TWRP. Does that get rid of all the TWRP stuff and bring me back to complete stock, so I can go back to rooting everything again? I'd be happy to do so, since right now I'm sitting with nothing but a dead Android.

Thank you so much for your help in advance!
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.