Not to claim that I know anything, but I think if you try

packages = with pkgs; [

Instead of

packages = [

You can remove the pkgs. before all the package names.

So instead of

packages = [
 # Awesome programs
  pkgs.firefox,
  pkgs.spotify,
  pkgs.discord,
  pkgs.slack,
  pkgs.zoom,

It would just be

packages = with pkgs; [
 # Awesome programs
  firefox,
  spotify,
  discord,
  slack,
  zoom,