Hey, this is a minor issue and would've been solved way easier and not even raised as an issue but a smarter brain than mine. However when copying and pasting and getting incredibly frustrated that I was constantly getting pi400kb.service fail no matter what I was doing I finally noticed that the script "ExecStart=/usr/sbin/pi400kb" was supposed to be "ExecStart=/usr/bin/pi400kb" which was causing the service to always fail because there is no path /usr/sbin/...

Hopefully in the meantime this is useful to others (all 12 of us) to edit the pi400kb.service before saving until this is sorted.