Yes, you can still file your 2015 tax return. Simply order the TurboTax software and install it on your computer. TurboTax Online for tax year 2015 is no longer available.
If you're expecting a refund, you have until April 15, 2019 to paper-file your 2015 return and claim your tax refund. After that, all unclaimed 2015 tax refunds end up in the U.S. Treasury.
Please see the following FAQ article for additional information. https://ttlc.intuit.com/replies/3301607