There's nothing above it. But I can try lowering the platform
When a platform is involved, that will cause a route fail sometimes because of the lack of headroom even when you can see nothing there. One way to test for that is have a sim go to the bottom of the stair then click "Go Here" on the platform in live mode. If the sim goes up the stairs to the platform, then click on the floor above and see if the sim goes there. That will tell you which part of the stair has the problem.
There is also some type of bug these days where stairs will either not work from the get-go or stop working. The usual fix for that is to hop into build mode and replace the stairs. It happens a lot in residential rentals and sometimes in other venues.
If you still can't figure it out, I am willing to take a look at the build and see if I can find what is causing the problem. All you need to do is upload the lot to the Gallery and reply here with the name of the lot and your userID if different than what you use here.
Hope this helps.
It is in the gallery. My user is morisalarson and its the most recent build.
