noob needs help

I get this specific error every time I use a skybox that has HDR properties like sky_gravel_01 (the one I want to use) according to the sky list on the developer wiki. I don't get this error when I use a non HDR skybox like sky_badlands_01 (the one I've been using to avoid the error thus far)...

I’m planning to start work on a Payload map, and with the things I’ve seen done with VScript, I have a question. Is it possible to have NPC’s spawn which are hostile to one team, lets say RED for this example but aren’t hostile against BLU? I don’t mean tf_bot_add command, but bots similar to...

I wanted to give my Brush a texture like like in Crash Video and it doesnt work, I have no idea and can i get please get some help? And when i load the map I only see the void.