Sims 3 won't launch

[LunaKittenRS]

I played Sims 3 earlier today with no problem. But as soon as I installed the Late Night Expansion, it won't launch at all now. It just gives me this message:

Please don't tell me I have to reinstall this crap. It took hours last time.

[puzzlezaddict]

@LunaKittenRS  It looks like your post was just moved from another section, so sorry about the delay in responding.

If you haven't uninstalled anything yet, you don't need to do so now, at least not before trying some less time consuming fixes.  The first is to create an alias for Late Night in your Applications folder, alongside the alias for the base game:

https://bluebellflora.com/sims-3-packs-not-showing-as-installed/

There is a second workaround that may be necessary, depending on which packs you have installed.  Four of them (aside from Late Night, the others are Ambitions, Fast Lane, and Outdoor Living) don't install correctly, and you may need to move some files around manually.  The key is that the folder for your newest pack, by the date EA released it, needs to contain folders from all the other packs.  The folders from the other packs aside from the problem ones will transfer over automatically, so you don't need to worry about them.  This video shows you how to copy over the correct files:

https://answers.ea.com/t5/The-Sims-Mac/CURRENT-ISSUE-Sims-3-Unknown-Error-in-El-Capitan-Sierra-High/...

So, if Late Night is your newest pack, and you haven't installed Ambitions, Fast Lane, or Outdoor Living, you can skip this second step.  If you do own Ambitions or Fast Lane, they're older than Late Night, so their files would need to go into Late Night's folder.  If you own a newer pack than Late Night, you'll need to copy Late Night's files into the folder for your newest pack.

If you have any questions, feel free to ask; just list the other packs you own, so I can give more specific instructions.

Edit:  @LunaKittenRS  I've moved your post to the Sims/mac section.  Please let us know if you still need help.