If you've installed Ambitions, Late Night, Fast Lane, or Outdoor Living, the Unknown Error is to be expected. There are two fixes, and you may need to apply them both (the video and the first link after it). If those don't work, the recommendation to set up an alternative admin account may (the next link).
https://answers.ea.com/t5/The-Sims-Mac/CURRENT-ISSUE-Sims-3-Unknown-Error-in-El-Capitan-Sierra-High/...
The process of uninstalling TS3 can sometimes be incomplete, leaving traces of the game on your computer that interfere when you try to launch. With multiple uninstalls and reinstalls, it may be best to do a clean uninstall before proceeding. Install just the base game first, then manually patch to 1.67 using the digital download version of the Super Patch and test to see if the game is working before installing any packs.
https://bluebellflora.com/how-to-do-a-clean-uninstall-and-reinstall-on-a-mac/
https://bluebellflora.com/sims-3-manual-super-patch-for-mac-1-67/
All newer mac OS are unsupported by TS3, as are the newest graphics cards. However, some combinations of card and processor make running the game particularly difficult or even impossible. If the above solutions don't solve your issue, you can post your specs here and someone will try to help you. If the Unknown Error persists, editing your .sgr files could help, but it may be that there is no way of getting TS3 running on your computer.
https://answers.ea.com/t5/The-Sims-Mac/CURRENT-ISSUE-The-Sims-3-Issues-playing-on-newer-post-2013-Ma...
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I don't work for EA. I'm just trying to help fellow players with their games.
