You can prepare a 2018 return but NOT with Online TurboTax. To prepare a 2018 return with TurboTax, you'll need to use 2018 desktop software (CD/download) installed on your computer.
And it cannot be efiled. It has to be printed, signed, dated, and mailed.
Online TurboTax is only for 2019 returns.
You can purchase 2018 desktop software as a download at this page and install it on your computer:
https://turbotax.intuit.com/personal-taxes/past-years-products/
Minimum system requirements for Windows version:
https://ttlc.intuit.com/questions/1901571-minimum-system-requirements-for-turbotax-windows-software
Minimum system requirements for Mac version:
https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software
Or did you already start a 2018 online return last year prior to the 2018 online shutdown, and want to finish that one without starting over? If so, you can download the *.tax2018 data file from your account and finish it in the desktop software. If you already paid for your product previously, TurboTax Support should furnish you the software.
The 2018 return cannot be efiled. It will need to be printed, signed, dated, and mailed. It's a good idea to mail by a method that has tracking, such as certified mail, priority mail, etc.
If you have both Federal and state returns, you need to mail each one separately to its respective tax agency.
If you are due a Federal refund, it takes about 6 weeks for the IRS to process a mailed return and deliver the refund.
