Let's perform some troubleshooting steps so you can track your mileage successfully in QuickBooks Online (QBO).
There are times that the browser is full of frequently-accessed page resources, thus causing websites, like QBO, to act weirdly. Let's try logging into your QBO account using a different or a private browser.
If you get the same result, I suggest tracking your mileage using your Android phone. For more guidance, you can check out this article for the detailed steps and information: Android Mileage Tracking.