I got this error in tool and I went back into max 6 and correcteda ll of the errors except for the open edges by the connection points of the tunnel to the skybox. And I still get this same error:

Here is what my max screen looks like for a reference:

I am no n00b and I have already searced around, so please if you have any suggestions or comments pointing out where I did somehting wrong, please let me know.