Hi there, user29895.
If you're referring to the Mileage in Quickbooks Mobile App, you can check if there is an update for the program. After updating, restart your phone and check again if you can already see the Mileage section.
For more information, you can check this article in troubleshooting this in Mobile Application: Android mileage tracking troubleshooting guide and Fix Mileage Tracker for your iPhone.
Also, this article help you track mileage in QuickBooks Online.
Let me know if you still cannot locate the MAileage section after doing the steps in the articles.