I've been trying to install iOS simulators 8.3 & 8.4 on Xcode 6.3.2 an 6.4 beta respectively. In the first case is not even an option to download this simulator while in Xcode 6.4 it shows an error "Could not download and install iOS 8.3 Simulator. Authorization is required to install the packages". I've searched the web but I could not find anything so I tries reinstalling Xcode from Apple developer website as well as from the app store with no change at all. The only simulator I can use is 8.2. I cannot install any other version. Any help would be really appreciated.
This is the error on Xcode 6.4 with no additional option of iOS 8.4 Simulator
These are the simulator options for me to download which result in that error
These are my options in Xcode 6.3.2 with no option for iOS 8.3
I found a workaround on the Apple forums. If you run Xcode using sudo the simulator downloads and installs work just fine:
sudo /Applications/Xcode.app/Contents/MacOS/Xcode
Note: Once the downloads and installs are complete you should quit Xcode and start it normally.
The iOS 8.3 Simulator runtime is includes in Xcode 6.3 (which is why it isn't offered as an additional download).
The iOS 8.3 Simulator runtime should be downloadable from Xcode 6.4, as you show. Your dialog box reports that there was an authentication problem when attempting to install the package. You need to provide administrator credentials when installing the package.
The reason you are getting your "iPhone 6 runs iOS 8.2 which is lower than -project name- minimum deployment target." errors is because you are selecting an iPhone 6 with iOS 8.2 on it and not an iPhone 6 with iOS 8.3 on it.
Use 'xcrun simctl create' or Xcode's Devices window to create the iOS 8.3 devices you need.
来源：https://stackoverflow.com/questions/30872040/xcode-6-4-could-not-download-and-install-ios-8-3-simulator