working with navmesh in different ground levels

I need some help regarding navmesh when working with ground of different levels;

as you can see here, my site has many different levels, even higher parts in the middle. The attached photo is my attempt to add navmesh to the sunken area in the middle because when I do VR preview, i am standing on a higher level, how can I fix this? I am trying to mive it down but it disappears.