A 2018 tax return prepared using TurboTax could only be completed using the TurboTax desktop editions. The 2018 tax return could only be printed, signed, dated and mailed by the user of the software to the IRS. The 2018 tax return could not be e-filed.
The TurboTax online editions are Only for tax year 2019. If you used the 2019 TurboTax online editions to file your 2018 tax return, then the IRS does not have a 2018 tax return on file for your name and Social Security number.
You will have to amend the 2019 tax return that was filed and enter your 2019 tax data on the return.
See this TurboTax support FAQ for amending a 2019 tax return using the online editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/01/27577
To complete and file a 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2018 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2018 tax return can only be printed and mailed, it cannot be e-filed.
