You can choose between mileage or actual expenses. In order to claim vehicle expenses and mileage, you need to add a vehicle to your business; here are the steps in TurboTax Online:
Navigate to Federal > Wages & Income > Self-employment income and expenses
Edit your business
Choose add expenses for this work
Choose Vehicle and Continue
Vehicle now appears in the list of expenses, choose the pencil icon to the right of vehicle expenses
Answer the interview questions about the vehicle, and enter the related expenses