Can't Place buildings on custom map

[Lolkat]

Today I attempted to make a custom map and I got almost everything to work fine, it loads in game, units can move. The only thing I can't do is place buildings on the ground. They will always show up red and say "Invalid Position" when I try to place them. Am I missing a certain brush or entity?

[Sandern]

Did you generate a navigation mesh? "nav_generate"

https://developer.valvesoftware.com/wik ... ion_Meshes