@Gegahex Try enabling Full Disk Access on your Mac, again in the new user account. Open System Preferences > Security & Privacy > Privacy, click Full Disk Access, and add both Origin and Sims 3.
If that doesn't work, the only other thing I can think of is reinstalling macOS. You wouldn't lose your data, you'd be reinstalling the current OS over the top. Here's a guide:
https://support.apple.com/en-gb/guide/mac-help/mchlp1599/mac
The other potential issue that came to mind is if you bought Sims 3 from a third-party source and happened to receive a bad product key, this might be the result. I've only seen this happen in Windows, not in macOS, at least as far as I know, but the symptoms there are similar (although not identical). So if you did buy from some other site, it may be worth contacting EA customer support and asking someone to look at the status of your base game product key, not just whether it's on your account but whether it's been blocked. This can happen after the fact too—just because the key worked before doesn't mean it always will.
https://help.ea.com/en/contact-us/
If you bought the game from Origin, this shouldn't be an issue.