@noseballs Are you running macOS Catalina? If so, that's the issue: Sims 3 is a 32-bit app, and Catalina is 64-bit only. There's no way around this error. But EA has announced a 64-bit-compatible version of TS3 for macOS, to be released some time this year. 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 (if you have at least temporary access to a Mac not running Catalina) to install Mojave on an external hard drive or USB stick. If you only just bought the game and don't want to wait for the new version, you can request a refund from EA.
https://help.ea.com/en-us/help/account/returns-and-cancellations/
