Sims 3 base game won't launch: 'Unknown Error Occurred'

@HannahLynn98  The reason you're still getting the unknown error is that a regular install, either a straight deletion or the kind done through the uninstaller that comes with TS3, doesn't remove the game's hidden library files.  You'll need to uninstall the base game again and then manually clean out those files.  Here's how:

https://bluebellflora.com/how-to-do-a-clean-uninstall-and-reinstall-on-a-mac/

Then install the base game only, and run the Super Patch (digital download version) to manually patch the game to 1.67.  The launcher doesn't do a good job of patching on its own.

https://bluebellflora.com/sims-3-manual-super-patch-for-mac-1-67/

If you still get an unknown error at this point, let me know.  Otherwise, you can start installing your packs.  There are four that don't install correctly (Ambitions, Late Night, Fast Lane, and Outdoor Living), and they need two workarounds, applied together, to fix the problem.  The first is simple: create an alias for each pack that doesn't already have one in Applications, alongside the alias for the base game.

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

The second fix is to copy a folder for each of those affected packs into the folder for your newest pack by the date EA released it.  (So that would be Into the Future if you have it, otherwise you can look up the release order.)  This video shows you exactly what to do:

https://answers.ea.com/t5/The-Sims-Mac/CURRENT-ISSUE-Sims-3-Unknown-Error-in-El-Capitan-Sierra-High/m-p/5725514#M5926

If you have any questions or would like further explanations, please feel free to ask.  It would be helpful to know which packs you're installing though, so I can describe the steps more specifically.

@puzzlezaddict  Thanks so much for the help!

I got the base game launcher to work, and then the High-end Loft Stuff Pack by following the "alias" steps in the third link you provided, but when I try to repeat the same steps for Supernatural, I keep getting the error.

I tried following the accepted solution provided here, but my "thumbnail" when I try to launch the game is Supernatural, which obviously already has the Supernatural folder in it, so I don't really know where I'm supposed to go from here.

@HannahLynn98  That's strange that Supernatural would be throwing the error; it isn't one of the packs that's affected.  (And you shouldn't have to make an alias for either it or HELS; the install process is supposed to take care of that.)  What other packs have you installed so far this time around?  Have you uninstalled any since the clean uninstall?

Nope, I just downloaded them on Origin - no install yet aside from the two mentioned. 

@HannahLynn98  Neither pack should need any workarounds; they're both supposed to install correctly.  So it's strange that you're getting an unknown error again.  Try restarting your computer and then launching TS3 directly from Applications, without opening Origin at all.  If that doesn't help, try bypassing the launcher.  Here's how:

https://bluebellflora.com/sims-3-how-to-bypass-the-launcher-on-a-mac/

If that doesn't help, do another clean uninstall, and then create a new user account (make it an admin) and install the game there instead.  Again, install only the base game, then run the Super Patch, and test.  If it works, you can install HELS and Supernatural and test again, without applying any workarounds.

@puzzlezaddict Got it to work with those two packs, but after installing pets, I'm getting the error again with the Supernatural thumbnail.

@HannahLynn98  This is still strange; it worked fine the last time I did a test reinstall.  Granted, that's been a few months.  Just to see, please try launching by double-clicking on the base game icon, and Pets if that doesn't work.

If you still get an unknown error, maybe none of the correct folders are copying over?  You can look for them easily enough.  Inside Applications\TS3, right-click on the Sims 3 base game icon and select Show Package Contents.  From there, the path is:

Contents > Resources > AddOns > Supernatural [show package contents] > Contents > Resources > transgaming > c_drive > Program Files > Electronic Arts

Inside Electronic Arts, what folders do you see, and are they aliases or full folders?  (If a folder has a little arrow in the lower left corner, it's an alias.)  Since Supernatural is newer than Pets, there should be a Pets alias inside, created the first time you tried to launch the game (via the Supernatural icon) after installing it.  Please list everything that's there.  If there's nothing Pets-related, you can try copying over its folder into Supernatural, as in the unknown error video I linked.  If there is an alias for Pets, right-cllick on it and select Move to Trash, then copy over the folder.

The other three folders were all alias' (base game, HeLS, & Pets). I deleted the alias Pets folder and copied over the full folder from Pets' 'Show Package Contents'. It's still giving me the error.

I apologize for this being messy.

@HannahLynn98  You're not being too messy, it's just strange to me that the game is being so stubborn, especially with packs that are supposed to be easier to handle.  If all the folders are in place, and you have an icon for each pack in Applications, then I honestly don't know why the game isn't working.  I take it bypassing the launcher didn't help?  On machines without much RAM installed, the unknown error can be due to the launcher reaching for more memory than is available, but that shouldn't be the case for you.

The only other thing I can think of at the moment is installing a newer pack than Supernatural, to see if that helps.  Maybe if a different pack is launching the game, whatever problem you're seeing now will no longer be in the picture.

Tried that with IP - still didn't work. I think I'm just gonna give up on it for now and just stick with TS4. 

Thank you so much for trying to help though!