$ sudo nixos-rebuild switch
building Nix...
building the system configuration...
downloading ‘https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz’... [10088/11731 KiB, 9902.8 KiB/s]
unpacking ‘https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz’...
error: Package ‘plex-1.5.5.3634’ in ‘/nix/store/ijd06p153j0ql6pf3wb7im7hqgp9pj43-nixos-unstable.tar.gz/pkgs/servers/plex/default.nix:78’ 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)