Unusable rooms

@handpalm 

It's kind of hard to tell what is going on from your pic.  I can see what looks like a kitchen below and assume that's the open area but can't tell for sure and I can't tell what the stairs are doing.  So, a couple more pics would be helpful.  I can think of a couple of things that would cause the route fail though.  I see you are using the short walls.  Take a look at any overhead lights in the room with the grey floor.  They may be hanging too low, and sims can't walk under them.  Another thing it could be is not enough headroom at the top of the stairs.  I think I see you've used a bent stair and sometimes in some situations, especially if a platform is involved, you can get a not enough headroom issue causing a route fail.

If you would be willing to upload the lot to the Gallery, I am willing to take a look at the build to see if I can figure out what is going on.  Just reply here with the name of the lot and your username if it is different than the one you've used here. 

Hope this helps.