Material not found

Material not found!: [texture directory/name], material [texture directory/name] not found

The texture path and/or name of the texture have not been properly specified or they simply do not exist the directories you've told Hammer they should be in. Check your directory paths.

You can also get a lot of these errors after having instanced another map into a map for which instances doesn't work. (Outside the Left 4 Dead 2 or the Alien Swarm games.) These errors will then persist until you remove the func_instance from your map and restart Hammer.