On digging further, it seems there is a service file present.
$ cat /etc/systemd/system/nix-daemon.service
ExecStart=@/nix/store/6chjfy4j6hjwj5f8zcbbdg02i21x1qsi-nix-2.3.1/bin/nix-daemon nix-daemon --daemon
But, still the
systemctl enable fails
$ sudo systemctl enable nix-daemon
The unit files have no installation config (WantedBy, RequiredBy, Also, Alias
settings in the [Install] section, and DefaultInstance for template units).
This means they are not meant to be enabled using systemctl.
Possible reasons for having this kind of units are:
1) A unit may be statically enabled by being symlinked from another unit's
.wants/ or .requires/ directory.
2) A unit's purpose may be to act as a helper for some other unit which has
a requirement dependency on it.
3) A unit may be started when needed via activation (socket, path, timer,
D-Bus, udev, scripted systemctl call, ...).
4) In case of template units, the unit is meant to be enabled with some
instance name specified.