EDIT: This question refers to teh pre-release version of Xcode 7, hence it may not be relevant if you are running the currently released version as some issues may have been
I got this error on with Xcode 7.0 release build (7A220). I was running on device too not using the simulator at all.
I did a full clean Clean build folder... option, close Xcode and then opened up Activity Monitor. I found a bunch of Ibtool processes running. I terminated all of these and restart Xcode.
All going again!
Another things I would check it's what command line you are using.
I had to use a different version and I changed them with :
sudo xcode-select --switch /Applications/Xcode8.app/Contents/Developer/
( You can also change it in the XCode/Preferences/Location )
I think my problem was that I was using the version 8.1
You can see the comand line you are using with:
xcode-select -p