Online TurboTax can only prepare a 2019 return at this point in time. For a 2018 return, if you want to use TurboTax, you'll need to use 2018 desktop software (CD/download). I'll also tell you another option at the end of this post if you want to prepare it manually with forms and instructions from the IRS.
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
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 would normally take about 6 weeks for the IRS to process a mailed return and deliver the refund. However, the IRS has not been processing paper returns the last few weeks due to the Coronavirus situation, so expect a longer delay as the work through the backlog.
OR another option is to download the 2018 forms and instructions for free from the IRS website and fill them out manually.
https://www.irs.gov/forms-pubs/prior-year
