I've been using Xcode 6.3 with El Capitan since El Capitan was released. I updated my Mac yesterday and it said that Xcode 6.3 isnt compatible with this version of OS X, i then downloaded Xcode 6.4 from the Mac App Store and the same thing happens.I then downloaded Xcode 7 beta 2 but it crashes everytime i open it.
Xcode 7 crashing and 6.4 not working
It would appear that Apple made a change in the beta OS X that prohibits the release versions of Xcode running.
There is a work around though...
See this thread...