@cece-bean
You can contact EA for a refund if you bought it within 24 hours
Sims 3 really isn't compatible anymore on most Macs, either due to the hardware or the version of macOS you are running. You also don't need Origin to run Sims 3, just to download it. In Origin go to Preferences then Installs & Saves then click on Browse Installers. A Finder window will open and you can look in the folders for the disk image. If it is there double click on it to open the installer. If it is not there then please reset Origin and try again. There is also a Mac/Origin help thread on this Mac forum which is definitely worth a read for more tips.
Please also read this thread regarding the Unknown Error message - http://answers.ea.com/t5/The-Sims-Mac/CURRENT-ISSUE-Unknown-Error-issue-in-Sims-3-in-El-Capitan-Sierra/m-p/5725514#U5725514