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:
Click on Expenses from the left panel.
Click on New vendor.
Enter the necessary information.
Click on Save.
Once done, you can now create the Mileage expense account and Product/Service.
Here's how you can create a Mileage expense account:
Click on Accounting from the left panel.
Choose Chart of Accounts.
Click on New.
Set up the Expense account details.
Click on Save and Close.
Then, kindly follow these steps in adding a product/service:
Click on Sales from the left panel.
Choose Products and Services.
Click on New.
Select Non-Inventory or Service.
Enter Mileage as the Name.
In the Sales information section, check I sell this product/service to my customers.
In the Purchasing information section, check I purchase this product/service from a vendor.
Enter a description and the cost you pay per mile.
Assign the mileage expense account you just created.
Select Save and Close.
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.