I managed to get it running, but what it does is create an extra volume and mount entry for it. If your boot volume can't be altered by the script, you have to do it by hand, and that probably requires booting in maintenance mode. The reason behind that is that they chose /nix as their store, and it's apparently hard-coded all over the place, and macOS 11 has locked /, so you can't have a simply directory anymore.

They do explain why they chose this option, and provide you with others (each with its own disadvantages). Nix seems to be the arch of package managers.