Stairs triggering nothing to work

FR125 I tested this in my game and had no issues with placing stairs, changing their width or saving and exiting to manage worlds.  I tested both with my mods in, in my main save and in a vanilla save.  So, it doesn't appear to be a bug for everyone, or at least not for me.  So, here are a few things you can try.

1. If you use mods and CC, move the mods folder to your desktop, start the game, load the save and see if it still happens.  If it doesn't, then it's something in your mods folder that you will have to find and delete or update.  If it does happen then....
2. Repair the game, delete the localthumbcache file and load the save.  If that doesn't fix it, then ...
3. Delete the localthumbcache file and start a new game and see if it still happens.   If that doesn't fix it, then ...
4. Create a new user folder.  Rename your existing The Sims 4 folder or move it to your desktop.  Start the game, create and place the mandatory sims then save and exit. Copy your saves folder from the old The Sims 4 folder to the new one.  Start the game and load the save and see if it still happens.  If it does, it's something in the save and you'd probably be better off abandoning it. 
5. If doing all of those things doesn't fix the problem, the only thing left is to do a full uninstall and reinstall of the game.  If you have to do that, move your existing The Sims 4 folder to your desktop first so you don't lose your saves.  

Hope this helps.