Forum Discussion
@brontetemps The issue is that Sims 3 is a 32-bit app, and Catalina is 64-bit only, so it won't run the game or even the installer. There's no way around this problem. But EA has announced a 64-bit-compatible version of TS3 for macOS, to be released some time this year. (We don't know when that will happen yet.) Here's the full announcement:
In the interim, your only options are to play on another computer, to install Windows on your Mac via Bootcamp or Parallels (probably not a good idea with a MacBook Air's small hard drive), or to install Mojave and the game on an external hard drive and boot into that when you want to play. Let me know if you'd like more information about these options. If you only recently bought the game and don't want to wait for the update, you can request a refund from EA.
https://help.ea.com/en-us/help/account/returns-and-cancellations/