I found the mullvad-daemon , nix-store files in /nix/store/svf..............-user-environment/bin/mullvad , I don’t know that’s it. I copied and pasted both files into ~/.config/systemd/user/ then I open the terminal from ~/.config/systemd/user/ , I typed systemctl --user enable mullvad-daemon.service, it says : Failed to enable Unit mullvad-daemon.service does not exist