error: Package ‘hplip-3.19.1’ in /nix/store/via1aa5d96v2qz4yyg978x51hnampjaz-nixos-19.09pre179307.bc94dcf5002/nixos/pkgs/misc/drivers/hplip/default.nix:217 has an unfree license (‘unfree’), refusing to evaluate.

a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnfree = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnfree = true; }
to ~/.config/nixpkgs/config.nix.

(use '--show-trace' to show detailed location information)