If you did not file a 2018, you cannot file it with the Online version.  The Online version is only to file the current years tax return (2019)

Prior year returns cannot be e-filed with TurboTax.  To file your 2018 return with TurboTax you will need to use the 2018 Download software (or go to the IRS website for prior year forms).  To purchase the software from TurboTax go to the following website.

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

----------------------------------

If you filed a 2018 return & it was accepted, you would need to amend your 2018 return to fix or change information.

How do I amend a 2018 return in TurboTax?

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27579