# nixos-rebuild switch
building Nix...
building the system configuration...
error: Package ‘discord-0.0.5’ in /nix/store/w631bj96k7pnxqpp3zf8hhi0wdgdxd8g-nixos-unstable-18.09pre147850.2428f5dda13/nixos-unstable/pkgs/applications/networking/instant-messengers/discord/default.nix:55 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)