There are some possible reasons why mileage feature won't show up on mobile phone app, one could be an outdated version of mobile app. You can follow the steps given by my colleage jamespaul to update the QuickBooks Mobile app for andriod and IOS devices.
To track your mileage using your mobile phone, you’ll have to click the Miles/Mileage from the left menu. This way you can add your trips and use the mileage tracker in mobile app.
To turn on the mileage tracker in QuickBooks Mobile app, here’s how.
Toggle Auto-tracking on.
You may also use these articles to guide you what do if your mobile stopped tracking mileage: