If there isn't a "Motion and Fitness" detector setting on your iPhone, it might be because you haven't updated your phone to iOS 15. Check if you've installed the latest iOS update by:
- Going to settings
- Tapping on General
- Selecting software updates
If there are any pending updates for iOS 15, update your phone's settings to enable the 'Motion and Fitness' feature. Once done, uninstall and reinstall the app.