Basically you are supposed to use
<channel-name>.<package>. Since the listing shows that you have
nixpkgs channel (which is default on non-NixOS) you should be using
nix-env -iA nixpkgs.some-package, not
It’s a bit confusing, but the difference is largely historic. For
nix-env, you can use
nix-env -f '<nixpkgs>' -iA some-package if you want your command to work on both setups. I have this alias set up:
alias nixpkgs="nix-env -f '<nixpkgs>'"
The new command
nixpkgs.some-packages works for both NixOS and just Nix, however it’s still lacking in features and doesn’t do what
nix-env does yet so you still have to use
nix-env for now.