The 2016 software can only prepare a 2016 return. Plus the 2016 software has just been released, so it is not a finished product. There will be updates and revisions in coming weeks, especially if any tax laws are changed at year's end.
A 2015 return would have to be prepared in 2015 software. You can purchase it as a download from TurboTax at the page linked below, or you may be able to find it at an authorized reputable retailer like Amazon.
https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2015 return cannot be efiled. You will need to print, sign, and mail the return.
