i get this error

WARNING: NavVolume generation is skipped. Flying units might not behave correctly. This might be due to there is no nav_volume brush in the level, or the generation is skipped purposely by skipping -navvolume option.

This shouldn't really be a problem if you don't need zombies to walk around and since(at least for me) this doesn't stop the compile.

If you are looking to fix this, you just need to add a nav mesh to your map. This is used to let zombies walk around for example if you are doing a zombies map.

To do this just go into entity browser and search navmesh and then drag it into your map and make sure it covers everything.