In QuickBooks online use the search button on the top right side of webpage (magnifying glass) and type mileage.  When you search it "Jump to Mileage" will pop up select that.  Once in the Mileage page select the year you need the report from on the drop down beneath the page title "Mileage" on the top left corner of web page.  Last step on the upper right side of page you'll see a green button that says "Add trip" with drop down, select drop down and click download trips.

Hope that helps, I had the same issue.

Regards,