-

error: while evaluating the attribute ‘x86_64-linux’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/lib/attrsets.nix:151:41:
while evaluating anonymous function at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/nixos/release.nix:85:49, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/lib/attrsets.nix:248:43:
while evaluating ‘hydraJob’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/lib/customisation.nix:139:14, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/nixos/release.nix:85:57:
while evaluating the attribute ‘outPath’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/lib/customisation.nix:155:13:
while evaluating the attribute ‘outPath’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/customisation.nix:127:40:
while evaluating the attribute ‘buildCommand’ of the derivation ‘options-json’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/nixos/doc/manual/default.nix:78:5:
while evaluating ‘substFunction’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/nixos/doc/manual/default.nix:12:19, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/nixos/doc/manual/default.nix:23:48:
while evaluating the attribute ‘default’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/options.nix:98:37:
while evaluating ‘scrubOptionValue’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/options.nix:113:22, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/options.nix:98:47:
while evaluating ‘isDerivation’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/attrsets.nix:252:18, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/options.nix:114:8:
while evaluating the attribute ‘default’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/nixpkgs/nixos/modules/services/misc/plex.nix:60:9:
while evaluating the attribute ‘plex’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/pkgs/top-level/all-packages.nix:2762:3:
while evaluating ‘callPackageWith’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/customisation.nix:93:35, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/pkgs/top-level/all-packages.nix:2762:10:
while evaluating ‘makeOverridable’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/customisation.nix:54:24, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/customisation.nix:97:8:
while evaluating anonymous function at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/pkgs/servers/plex/default.nix:1:1, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/lib/customisation.nix:56:12:
while evaluating ‘mkDerivation’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/pkgs/stdenv/generic/default.nix:93:5, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/pkgs/servers/plex/default.nix:5:1:
while evaluating ‘throwEvalHelp’ at /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/pkgs/stdenv/generic/default.nix:118:23, called from /nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/pkgs/stdenv/generic/default.nix:167:21:
Package ‘plex-0.9.14.6.1620’ in ‘/nix/store/639jxmwp7a19by3qh2wf0vlb699383iy-nixos-16.03pre75174.d0e3cca/nixos/pkgs/servers/plex/default.nix:61’ 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` or any other Nix command you can add
  { allowUnfree = true; }
to ~/.nixpkgs/config.nix.

Error that happens when I run:
nix-build -Q --no-out-link '<nixpkgs/nixos/release.nix>' -A options --show-trace