active developer path ("/Volumes/Xcode/Xcode.app/Contents/Developer") does not exist, use xcode-select to change

Try the following command:
sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer

If the above command works, do not read further :)

If you get the following error,
error: invalid developer directory '/Applications/Xcode.app/Contents/Developer'

Copy the XCode.dmg to Applications directory and double click it. Then copy the XCode to Applications directory and try the above command again.