A 2015 W-2 must be entered on a 2015 tax return. If you have already filed a 2015 return, you would have to amend it to add the W-2. If you have not filed, then you must buy and download the CD/download product to prepare, print and mail it. Here is a link to buy it: https://turbotax.intuit.com/personal-taxes/past-years-products/
You can't use the online product. It is only for 2016 tax return. A 2015 tax return cannot be efiled.