A rather super person called coreylamalva414 over at AHQ has come up with a fix for this issue. If you're running El Capitan
and have Late Night
installed but your game is not working and throwing up the Unknown Error
message then this fix is for you. You can find full instructions on how to apply the fix in the accepted solution (second post on page 1) in this post - http://answers.ea.com/t5/The-Sims-Mac/CURRENT-ISSUE-Unknown-Error-issue-in-Sims-3-in-El-Capitan-Sierra/m-p/5725514#U5725514
Don't forget to hit the 'Me too' button below the opening post if you're suffering from this issue. EA need to fix this!