Fucking fpsb wont let me respond to your message on my tutorial. seems like you dont have no sky and lightenvironment to have a correctly working cubemap. so dont let it all be in a box, but instead make the roof a sky and add an lightenv or/and and env_cubemaps.

Going around pasting links to interlopers is not helpful. Especially when if you had checked for yourself, you would have seen the compile logs don't have any useful errors.