I'm getting the same error with the Xcode 6 GM. It also applies to trying to run 7.0 or 7.1 simulators.
Has anyone encountered this and found a solution?
I found a workaround on the forum of Starling Framework: http://forum.starling-framework.org/topic/iphone-6-question
I simply used IntelliJ to do the .ipa build, then after the error (but successful .ipa build), used ios-sim with the unzipped Payload/_.app using lines like the following in the command line to launch the app running in the simulator:
ios-sim launch ../../Payload/_.app --devicetypeid com.apple.CoreSimulator.SimDeviceType.iPhone-6 or ios-sim launch ../../Payload/_.app --devicetypeid com.apple.CoreSimulator.SimDeviceType.iPhone-6-Plus
For me it worked using last AIR 15 beta 220.127.116.118 and Xcode 6.0.1. It works perfectly even debugging!
But if you use last Xcode 6.1 GM beta won't works. You should set the environment variable to use the device version that you want, just run this command in your terminal:
launchctl setenv AIR_IOS_SIMULATOR_DEVICE "iPhone 6 Plus"
I hope it helps