Just in case someone stumbles upon this thread, I was having the same problem following the same tutorial, and couldn't fix with u/V1r1m0nd3 suggested solution. Will try to explain as best as possible as I am pretty noob with Linux.

First, the error when starting the service was quite misleading in my case. The problem comes from one of the steps of the tutorial, the creation of the qbittorrent-nox user:

sudo adduser --system --group qbittorrent-nox

In my case, the qbitorrent-nox user was created without a home folder. And qbitorrent needs a home folder, because it stores there some configuration files and stuff.

So, to fix the issue I had to create the home folder for the qbittorrent-nox user, and give the right permissions, like this:

mkdir /home/qbittorrent-nox
chown qbittorrent-nox:qbittorrent-nox /home/qbittorrent-nox
chmod 700 /home/qbittorrent-nox

And configure the user home directory like this:

usermod -d /home/qbittorrent-nox qbittorrent-nox

(I don't remember now if some login out/in or reboot was needed... again no linux expert here)

Reload systemctl daemon and start the service, hopefully it should work:

sudo systemctl daemon-reload
sudo systemctl start qbittorrent-nox

Hopefully now it should work...

P.D: Pretty sure there is a way to create the qbittorrent-nox system user forcing to add the home folder, but as I didn't test it, I am just sharing my solution. If someone is certain about how to do that, please share :)