What should I do if, on my 1095-A, columns A and B are blank but column C is filled in? This appears on my paper 1095-A form, but TurboTax will not accept it.

You can leave Column A blank but Column B will need to have entries. Usually the second lowest cost Silver plan (SLCSP) premium can be found in Column B of your Form 1095-A, but sometimes it may be missing or incorrect. 

Click this link for the Marketplace calculator tool to determine what number should go in Column B and enter it there.