What do I do? If TurboTax states I need to enter a value for box 16 a from my W-2. However, I don’t have a box 16 a listed on my W-2?

If there's nothing in your W-2, try entering "0" or leaving it blank. If your state doesn't have state taxes (for example Florida) but have "FL" in box 15 and nothing in box 16, then leave both 15 and 16 empty.