Turn on location services for your mobile phone:
iOS: Settings Privacy Location Services Set to ON Android (Samsung 6.0.1): Settings Privacy & Safety Set to ON
Turn on your Strava app's location services:
iOS: Settings Strava Location Set to "While Using" or "Always"
Android: On by default.
When you go out for a ride, hit record and turn off your mobile phone's screen. Special thanks to user triton for sharing this information.