In Xcode 6.3, I got this error when trying to run an iOS project written in Swift on a real device.
App installation failed
c
For anyone still facing this Issue:
There are so many reasons and this is one of them as it happened to me.
I've faced the same problem and I was connected wirelessly and If you are trying to install the application in wireless connecting mode (connected through same network), this might probably because of poor internet connection. Check your internet connection and try again, it works fine...