Did you already file a 2015 return? Then you need to amend it to add to it. How to Amend
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed If you need to file an original 2015 return, Online is only for the current year so you have to buy the Desktop CD/Download program here
https://turbotax.intuit.com/personal-taxes/past-years-products/ Or you can get the blank forms from the IRS and do them by hand. Either way you will have to print and mail prior year returns.
