Xcode 7 with El Capitan unable to sign binary for export - can't press "allow" button
El Capitan does not allow software to click the "Allow" or "Always Allow" buttons for security reasons. If you are running software that intercepts mouse events such as MagicPrefs, this may register as a software click. Try disabling any software that reads mouse events, or using a standard USB mouse.
I found the answer here: https://forums.developer.apple.com/thread/25092and here: https://discussions.apple.com/thread/7305746
Not a complete solution but I was able to work around this by booting into Safe Mode and building from there. While in Safe Mode I also added the Xcode application to the keychain item and now it doesn't ask for permission anymore.