The navigation meshes define the surface where the centers of objects can be so from what I can see it looks like that tank has its center on the navmesh and everything looks fine.
That warning is a bit annoying though. Currently the linecast implementation may fail in some cases due to floating point errors. For this case you should be able to ignore it however, it will not impact the movement in any significant way.
I have an entry on my todo list to rewrite that code, but I haven't gotten to it yet unfortunately.