TurboTax online and e-file for 2018 ended on Oct 15, 2019. 2018 (and earlier tax years) can now only be completed using the CD/download software installed on a Mac or Windows computer. The return must be printed, signed and mailed.
2016-2018 software can be purchased here:
Prior year tax return cannot be e-filed using the TurboTax online editions.
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 2017 and 2018 desktop editions 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 envelope for each tax year being mailed.