First, flakes are "experimental", so you have to enable them. Back then there were like three slightly different CLI commands to do it, and it felt like none worked from like 5 tutorial tabs I had open, putting it `experimental-features =` into flake you are trying to switch to does not work obviously.
Then you hit the classic situation where your flake is not committed or staged, so Nix refuses to see it. And instead of telling you that, it prints this abomination of error message "error: path '/nix/store/0ccnxa25whszw7mgbgyzdm4nqc0zwnm8-source/flake.nix' does not exist" (https://determinate.systems/blog/changelog-determinate-nix-3...)
I would not wish learning Nix from zero on my worst enemy, and I say that as someone who uses nix-darwin, devShells, deploy-rs and so on every day. The UX/DX is really bad, but nothing else comes close to its capabilities.
Sorry for rant but without flakes I would not make it.