react-native run-ios doesn't run: Application is unknown to FrontBoard?

try with another simulator if it works you need to reset your simulator from simulator menu

  1. Simulator -> Reset Content And Setting if you have Xcode 9 it will be from Hardware -> Erase All Content and Settings
  2. clean from Xcode menu Product -> Clean and run your project

As mentioned by Gavin Owens on Github, I followed those steps and it worked.

Just delete ios/build from iOS directory and after that run command react-native-run-ios

use following command

rm -rf ios/build then running react-native run-ios