nixpkgs was not found

[techflashYT]

When I try to do anything with it, when I get to the part where you need to run nix-env, it gives this:

error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)

    at /home/XXXXX/nixGL/default.nix:18:17:
        
        17|    # able to access the nvidia drivers.
        18|    pkgs ? import <nixpkgs>  {
            |
        19|       config = { allowUnfree = true; };
(use '--show-trace' to show detailed information)

I tried a lot of different things, and it just doesn't seem to work. Any ideas?