When I build my mod and run it after building it, I come am spawned into the game and all the walls around me are invisible, and my knife does no damage; also take no damage when hit by zombies. However, the pistol does kill them. I've posted an image below showing the issue, please respond with an answer if you have any ideas to why this could happen.
Thanks!
https://giphy.com/gifs/jkYpTMF13O1w5y0znO/html5

No idea about the knife. Try using a different texture on a wall.
Check the properties I think there's an invisible check button. Also what kind of brush did you use. Maybe you used the wrong one. Can you walk through the walls?

I figured it out. Apparently the skybox was all screwed up so I just copied the skybox from a new map and pasted it into the current one. It works now, thanks though!