This is the biggest Mac/Sims 3 issue at the moment, apart from the unsupported graphics cards issue which EA STILL haven’t fixed after nearly 4 years…
The Unknown Error is a magical mix of Late Night and/or Ambitions trying to be installed onto a Mac running macOS 10.11 El Capitan or 10.12 Sierra. It’s that simple. We have a thread running over at AHQ where people are posting about it and yesterday a rather super person called coreylamalva414 posted a solution which appears to actually work (go and show him some love if it works for you 💚). His instructions can be found here. Good luck!
Edit – coreylamalva414 has uploaded a very helpful video to walk you through the fix:
Threads are starting to pop up on the forums with people moaning that their Sims games will not work properly, or at all, since they have updated to macOS Sierra beta. What a surprise! The operating system is in beta which means it will be buggy as hell and apps may not work properly. If you’ve signed up for the public beta it means that you agreed that stuff may not work properly on your Mac under Sierra and you should be reporting bugs and issues to Apple via the Feedback Assistant. You will have also been prompted to use Time Machine to back up your Mac before installing the OS – if you didn’t then give yourself a slap.
If you now can’t play because you opted into the beta program may I suggest you wipe and reinstall El Capitan on your Mac. Alternatively you could install El Capitan/Yosemite/Mavericks onto an external drive and play the Sims on there. Or you could wait until Apple release updates and hope your game works again. Even when Sierra is finally released in a few months time there is no guarantee that EA will support it.
No 3rd party apps are supported under beta versions of any operating systems. Maybe next time stick with the stable OS you are already running?