You can still complete your 2015 prior year tax return using TurboTax, You will need to purchase a CD/Download software for the 2015 tax return, Turbo Tax online can only file for the current tax year (2016).
Below is the website were you can purchase the prior year software to file your 2015 taxes:
https://turbotax.intuit.com/personal-taxes/past-years-products/