The TurboTax online editions are only for tax year 2017.
To complete prior year tax returns using TurboTax you would need to purchase and download one of the desktop editions for each tax year needed from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
Prior year tax returns can only be printed and mailed, they cannot be e-filed.
When mailing multiple tax years use a separate envelope for each tax year.