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:
Another issue that’s cropped up for Mac users. When trying to download and install University Life they are being notified that the EP has not yet been released. This seems to be related to specific product codes when purchased through Origin.
If you are experiencing this please contactEA customer support to request a replacement code or wait for EA to hopefully release a fix through Origin.