@callmewhatever12 Some custom stairs require a script to work correctly, so please check the creator's notes and if they mention that.
Just in case it's something else, because The Sims 2 can be temperamental sometimes, please test your game in a fresh user folder. Go to Documents\EA Games\ and move your The Sims 2 or The Sims 2 Ultimate Collection folder to the desktop. After you launch the game, a fresh folder will be generated without any modifications; play for a bit to see if it crashes. You can delete it and move your old one back once you know the cause of the crash.
Since reinstalling the game, have you used Graphics Rules Maker and/or 4GB Patch? These tools should help the game run smoothly and lower the risk of crashing on modern systems.