Unable to run app in Simulator XCode 6.0
iOS Simulator -> Reset Contents and Settings
Worked for me!
iOS Simulator -> Reset Contents and Settings... -> Reset
As described in my answer to the queation that this is a duplicate of, this error is one manifestation of an install/launch race condition bug. The suggestions you find on this forum suggesting that you should "Erase Contents & Settings" or "Clean" from within Xcode are not correct.
You simply have to try again and hope you don't loose the race on the next launch. Alternatively, you can just "finger launch" it from within the booted simulator.