How do I fill out the vehicle depreciation portion if I voluntarily surrendered my vehicle in 2019?

Indicate in TurboTax under Vehicle Expenses for your business that you 'stopped using your vehicle in 2019' and the date. 

Since you have 'converted the vehicle to personal use' enter $0 for a Sale Price in the 'Sale of Business Property' section. 

For Depreciation Taken, if you have claimed Actual Expenses for this Business Vehicle, check your Form 4562 from your 2018 return for the amount of prior depreciation.

If you have used Standard Mileage, depreciation is based on total number of business miles claimed.  If you have claimed 2000 business miles over the years, for example, multiply 2000 times .25 to get an amount to enter here for depreciation.

Since the Sale Price is $0 you won't have a Gain or Loss to report.