Did you already start a prior-year online return that you need to download and finish? Or do you need to start from scratch?
Either way you'll need to purchase the desktop software (CD/download) for that year. If you already paid online, TurboTax Customer Support can furnish you a download.
Otherwise, here's where you can purchase the prior-year software directly from TurboTax. You may also find it at a reputable authorized retailer such as Amazon.
https://turbotax.intuit.com/personal-taxes/past-years-products/
A past-year return cannot be efiled. It has to be printed, signed, and mailed.