I am assuming that this question pertains to a self-employment activity.

If you have expenses due to self-employed business expenses, you need TurboTax Home & Business in Desktop or TurboTax Self-Employed online.

You will not be able to enter your actual vehicle expenses until 01/29/2020, as the IRS is still working on finishing up Form 4562 (Depreciation and Amortization). After that date, please follow the below guidance to enter these expenses.

Please see the steps below to enter your expenses in TurboTax Online. 

1. Select the Federal Taxes tab
2. Scroll down to Self Employment Income and Expenses and click Review (this assumes that you have already entered your self-employed income)
3. Click Review again next to the specific self-employed activity and proceed through the interview until you reach the screen that says: What kind of expenses did you have for ______?
4. Check the box that says Vehicle and continue through the other expense checkboxes. Click Continue.
5. On the screen that says Here’s your ______ info, scroll to Expenses, and click Start next to Vehicle.
6. Proceed through the interview, enter your vehicle information, and enter your miles.
7. When you get to the screen that says Based on the miles you drove, your standard mileage deduction for this [vehicle] is:, scroll down and click the button that says I'll enter my actual expenses because my higher repair, gas, and insurance expenses might get me a bigger tax break.

I also have not been able to access Actual vehicle expenses in self-employment Turbo Tax, only mileage.  A note says it will be available once Form 4562 is available on January 21st 2019, the answer above says Jan. 29th.  I keep checking and clicked the box to  email me when the form is ready. Today is Feb. 6th and it's still not available. Am I missing something or is everyone else dealing with this hold-up on form 4562 too?

Federal individual forms availability says that the 4562 is currently available.

I used TurboTax Online Self Employed and was able to enter actual vehicle expenses to override the standard vehicle mileage.  You will want to try both options to see which works best for you.

This help and this help may be useful to you.