September 2024 update failing to install

SianneShep  Try repairing the game instead.  Clear the EA App's cache first, again.  The option to repair is under Sims 4 > Manage.  A repair would give you a fully-updated game.

If this doesn't help, would it be a burden to reinstall the game?  I would actually suggest you NOT uninstall, in case the reinstall process is broken too.  If you have the space, you could simply rename the Sims 4 folder that contains the game's program files, tricking the App into thinking it's no longer present.  You'd need to close the App and kill the EABackgroundProcess in the Task Manager first, and this only works if your computer has enough space.

If you do have the space and reinstalling wouldn't be too difficult, create a new folder on the root level of the drive of your choice, e.g. C:\Games, NOT C:\Program Files\Games, and install Sims 4 there.