Unable to run app in Simulator: Xcode beta 6 iOS 8

I solved it following these steps:

1. Open Xcode 6 beta
2. Go to the menu Xcode > Open Developer Tool > iOS Simulator
3. Even if an error dialog shows up, you still would have access to the iOS Simulator's menu
4. Select Hardware > Device > Manage Devices
5. Click on the little + sign at the bottom
6. Add (if missing) all the devices that you want, or delete and recreate the ones malfunctioning.
7. If anyone of the simulator is not working then right click on it and delete it and then re-create it again

• Make sure you choose Xcode 6 as Command Line Tools in Preferences > Locations Section

• Make sure you open Xcode 6 from Applications not from the setup file.

• Finally restarting Mac solve the problem in my case.

After I found some solutions on the web, a combination of procedures worked for me:

1. Close Xcode and iOS Simulator (and all related applications)

2. Install (or reinstall) Xcode6-Beta to Applications folder (direct from the .dmg file)

3. Open Xcode6-Beta and go to Xcode -> Preferences -> Locations -> Command Line Tools and select "Xcode 6.0" (be sure that you've selected the one in Applications folder)

4. Launch the iOS Simulator and go to iOS Simulator -> Reset Content and Settings...

5. Close all the applications and restart your mac (I unchecked "Reopen windows...")

6. Open Xcode6-beta and test the iOS Simulator

Good luck

I'm using Xcode 6 beta 2 and has problem (error code = 4 in my case) running iPad simulators. I think I had the issue because I'm running dual Mac OS boot and the Xcode is launched from another startup disk

I did both steps recommended by @ayalcinkaya and @apascual, and this worked for me.

• Change the "Command Line Tool" to Xcode BETA2 (from 5.1.1 in my case) in "Xcode->Preference->Locations"
• Open Xcode 6 beta 2
• Go to the menu Xcode > Open Developer Tool > iOS Simulator
• Even if an error dialog shows up, you still would have access to the iOS Simulator's menu Select Hardware > Devices, and switch between iPad devices and this apparently made the simulators reconfigured
• While the simulator is running, run the app with the selected simulator.
• After going through this manual process once, everything is fine.

This happens most of the times because another instance of the simulator is running in the dock.

Kill the other instance & things should work fine.