When I click run in Xcode, that is the message that pops up, but it worked a few days ago... I tried resetting the simulator settings but that did nothing. I have read that iOS simulator doesn't work on El Capitain, but that doesn't explain why it worked a few days ago and then randomly stopped working.
Fixed the issue, the ANSWER is UPDATE. People said this but I didn't have any available updates, but after a few days an update showed up in the appstore and after updating OSX, the simulator ran fine.