start with nav_mark_walkable to mark a walkable spot at your feet. Do the same for other walkable spots [though this map is very basic on those].

Then nav_generate

However, I have yet to be able to generate a navmesh even with this as it takes such a long time for no apparent reason.

I also believe certain npcs [usually of the nextbot variety] automatically ask you if you want to generate one if there isn't one.