Okay, now I found out that this is the very reason why you should install Node.js and NPM via NVM. I have no idea what NVM really is, but this worked:

curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.35.3/install.sh | bash

And after relaunch of Terminal:

nvm install stable npm install -g @quasar/cli

So I think those are the steps that you should have in your instructions instead of just

npm install -g @quasar/cli