Unfortunately, TurboTax Online is only available for the current tax year (2018) so you will need to purchase and download the software for both 2016 and 2017 to prepare Tax Returns for those years. These returns will need to be mailed in as only the current year can be E-filed through TurboTax.
Check out the TurboTax FAQ below for more details: https://ttlc.intuit.com/replies/3301770
