So you've run
$ react-native run-ios
and you got:
Command failed: /usr/libexec/PlistBuddy -c Print:CFBundleIdentifier build/Build/Products/Debug-iphonesimulator/MYAPP.app/Info.plist Print: Entry, ":CFBundleIdentifier", Does Not Exist
Just stop running that command again and again and try to build the project directly from xcode, you'll grasp more detailed information.
This is fucking pain ...