Are you an employee?  Form 2106 is for Employee Business Expenses, which are no longer deductible in your federal return unless you work in certain professions.  They may, however, be deductible on your state return. 

You can go back to the Form 2106 mileage entry by following these steps:

1. On the Deductions & Credits screen scroll to the Employment Expenses section.
2. Click the Start/Update box next to Job Expenses for W-2 income.
3. On the screen, Any Vehicle Expenses or Sales, click the Yes box.
4. Continue through the screens, entering the requested information.

If you want to enter vehicle expenses for your business, please follow these steps:

1. Click on Income and Expenses and then click the Start/Revisit box in the Self-Employment section.
2. On the Your 2018 self-employed work summary screen, click on Edit next to your business. 
3.  If you've already started to enter information on your vehicles, in the EXPENSES section click on Edit next to Vehicle
4. If you haven't started, on the Here's your [business] info screen, click on the box Add expenses for this work.  
5. On the next screen click the radio button next to Vehicle and click Continue at the bottom of the screen.
6. Continue through the interview to edit the mileage.