If you have been claiming Actual Expenses for your Business Vehicle, you originally entered it's Cost Basis when you started using it for business.

TurboTax has been calculating Depreciation for you each year, and will fill in the Prior Depreciation for you, which is subtracted from your original Cost Basis.

When Basis is subtracted from Sales Price (or Trade In Value), this determines Gain or Loss.

If you have been using the vehicle over 5 years, it is fully depreciated, and you can enter an amount equal to the Basis as Prior Depreciation, so there is no gain or loss. 

If you have been claiming Standard Mileage for your Business Vehicle, you will have to calculate the Prior Depreciation amount, based on total number of Business Miles, using the Depreciation Equivalent table below (screenshot).