Box 12 should be entered into TurboTax exactly as it is on your W-2. If there is nothing in box 12 on your W-2, leave it blank in TurboTax. If the program prompts you to enter a value you need to enter. Make sure "box 12" Letter Code is showing"-Select Letter Code-" for the letter code box and the amount box next to it should be blank.
I have attached a screenshot below.