trying it out in a vm not working

[Oliper202020]

[Zocker1999NET]

could it be that you had a typo in there or that your repository is not publicly accessible? there is no public repository on https://gitlab.com/salvesen1/nixos, at least GitLab requires me to sign in instead of showing me a public repository. And as Nix is currently not able to access non-public flake repositories (as far as I know).