Lets get technical

https://medium.com/p/adding-react-native-admob-in-your-app-c32f3be2de2d

If you have ever faced issues similar to the one below ld: warning: ignoring file ../lib/iPhone/xxxxx.a, missing required architecture i386 in file ../lib/iPhone/xxx.a (3 slices) Undefined symbols for architecture i386:   "_OBJC_CLASS_$xxxxxx", referenced from:       objc-class-ref in ViewController.o   "_OBJC_CLASS_$xxxxxx", referenced from: then most probably your project is set to test in a simulator. Change this to device, Command + B and magic!  Remember, never ignore the warnings and always read it completely. In this case, Xcode ignored linking the xxx.a file in the iPhone directory due to compatibility issues. I spent couple of hours on this one. Arrrrghhhh!