Each tax year is a separate TurboTax program.
A prior year tax return can be filed at any time if taxes are owed. For a tax refund the tax return must be filed within 3 years of the due date for the tax return. For tax year 2017 that would be on or before April 15, 2021. For tax year 2018 that would be on or before April 15, 2022.
To complete and file a 2017 and 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2018 desktop editions for each tax year required from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A prior year tax return can only be printed and mailed, it cannot be e-filed. Use a separate envelopes for each tax year being mailed.
