Navigation of AI

Hey All I was wondering and I’m a bit confused on what i should do to define a zone on the map that i have made that AI can access. Ive tried to use and build a nav mesh for map but crashes and when i doesn’t i don’t see anything. my main dilemma is do i even need such a thing because i want the AI to be able to go anywhere that the first person character can go. what defines that movement as far as what degrees of slope i can and cant get onto. it is quit a big natural map mountains, trees, rock, caves and things. So what i’m trying to understand is is there something the first person character has built in it that defines all the movement and allows me to walk on the ground not fall through and define where i can go that i can apply to an enemy so that they can do the same ?