Filing with a 2018 online version of TurboTax is now closed for the 2018 tax year. For TurboTax Online, 2019 is considered the current tax year while 2018 is now considered a previous tax year. 2018 returns that were started online but not completed and filed have been purged.
As a result, you will have to use a desktop (CD/Download) version of TurboTax to prepare and file (print and mail only at this point) your 2018 return.
See https://turbotax.intuit.com/personal-taxes/past-years-products/
