Prior-year products are not available in the "online" platform. You can purchase TurboTax 2016 by using the link below. You'll need to download and install the product you select (after purchase), print and mail your 2016 return(s). Prior-year returns cannot be e-filed, but must be filed by mail.
https://turbotax.intuit.com/personal-taxes/past-years-products/2016/
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***