If you donated a car, the recipient organization is required to give you a form 1098-C. You can't claim the deduction without it. Form 8283 is probably also required but Turbotax will generate this automatically.
The charity may not give you the 1098-C until after they dispose of the car -- for example, if you gave it for an auction to raise funds, they won't issue the form until after they sell the car so they can put that info on the form. (Your donation value is the selling price, no matter what the trade-in value might have been.) Since they may issue the 1098-C late, you may want to file your taxes without the car donation, and then amend your return to claim the donation when you get the form.
You would have to contact the organization to ask about the form, if you don't have it.