Help with poetry2nix nixpkgs overlay

Help
2

Because the overlay is not applied to the nixpkgs instance used by your NixOS config.

You need something like

{ inputs, ... }:
{
  nixpkgs.overlays = [ inputs.poetry2nix.overlays.default ];
}

For example, if you want to work this into some existing file like your nixos.nix:

{ inputs, pkgs, ... }:
{
  environment.systemPackages = [
    (pkgs.callPackage ./thePythonPackage.nix {})
  };

  nixpkgs.overlays = [ inputs.poetry2nix.overlays.default ];
}