Some cars will require a physical key to start, which will be called out in your reservation confirmation email. The car's key will be in a lockbox inside the car under the front passenger seat.
Once you unlock the car with your phone, a unique lockbox code associated to your reservation will display in the app. Use this code to open the lockbox and then use the key inside to start the car.
Remember to put the key back in the lockbox when you're done with your reservation!