Recording mileage is only available in QuickBooks Self-Employed. I can provide more details about this and provide you with a workaround available for QuickBooks Online.
When you say "record mileage" are you referring to your personal mileage? Or the mileage of your employees?
If you're referring to your personal mileage, you're only able to do this in QuickBooks Self-Employed. But if you're referring to your employee's mileage, you can freely record this on the online version.
You can create both a mileage account and products/service to record your employee's mileage. You'll also need to add them as a vendor to make it billable to them.
Also, in creating a bill, you're able to assign them to your job site. You can follow these steps in adding your employee as a vendor:
Once done, you can now create the Mileage expense account and Product/Service.
Here's how you can create a Mileage expense account:
Then, kindly follow these steps in adding a product/service:
Once completed, you can create the bill to track your employee's mileage. You can achieve this by clicking on the Plus (+) icon and choosing Bill.
You can also associate a job site on your bills by choosing a Location.
For further guidance, you may refer to this article: How to Track Mileage?
Don't hesitate to leave a comment below if you have any other questions. I'd be happy to help.