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:
https://turbotax.intuit.com/personal-taxes/past-years-products/