Only admin users can access the mileage feature in Quickbooks Online, gwynltd-gmail-co.
You'll want to ask the permission of the primary admin to add or transfer the access so you can track mileage in QBO. You can learn more about this by checking the article: Set up mileage tracking and vehicles in QBO and QBSE.
Also, I'll include these resources about what to do if you have problems tracking mileage, and recording business miles: