Unable to start: Please download Sims 4 Legacy Edition. WHY!? Please help!

@7e1dc5004a341ed25199d99c5c80f6aa  I've moved your post to the Sims 4 Mac section since it was originally in PC tech.  Please post the full specs of your Mac: everything under About This Mac including the OS version and except for the serial number.

Personally, I'd just stick with Sims 3, but I'm weird like that.  But for what it's worth, installing Sims 4 is usually not a problem at all either in macOS or Windows.  Losing data is also uncommon and mostly avoidable, but it's always a good idea to keep backups no matter what game you're playing.