I'm glad you've reached out to QuickBooks Community. Let me help guide you on how to track mileage in the mobile app.
To do this, here's how to automatic mileage tracking works:
To Turn it on, open the app on your mobile device, and tap the Mileage tab at the bottom of your screen (iOS) or tap Mileage from the menu (Android).
Switch the Auto-tracking toggle to turn on automatic mileage tracking. Make sure you also enable location services in your device's settings.
To Take a trip, we automatically detect your starting and stopping locations when you're driving for all of your trips, both business and personal.
To Identify your allowable expense, come back to Mileage after you take a work-related drive to categorize your trips. Just like you do with transactions, swipe right on your trip to categorize it as a personal trip, and swipe left to put it in a business category. Add a business purpose, assign the vehicle, save, and you're all set.