This may be due to different reasons. Check the below:
- Check the spelling of your storyboard name. Maybe it is different or capital letters may be different
- Check your bundle resources contain this storyboard from by selecting Target->Build Phases->Copy Bundle Resources. If not add it by the below plus button.
- Delete your app from the iOS Simulator. Then reinstall the app.
- Try to rename the storyboard and apply same name in Main storyboard file base name in your application plist file.