How can I stop auto-lock (iphone screen goes dark) while using apple map?

If you are using the Maps app directly on your iPhone and your iPhone is not connected to external power, then the screen is designed to dim/lock to conserve battery. You can prevent this by:

1. Connect your iPhone to power while using Maps on it directly. For example, while using in a car. OR
2. Open Settings ➔ Display & Brightness ➔ Auto-Lock: Set this to Never before a trip. You can set it back to the way it was after the trip is over.
3. Furthermore, if you want to suggest this feature, that you like, to Apple then please leave feedback here ➔ Product Feedback - Apple 

Axel F.