The current online software is ONLY for 2019 returns --which is what everyone is now preparing for the 2019 tax year that just ended. You cannot use online for any other year. And you have until Oct 15, 2020 to efile your 2019 return.
Prior year returns can only be done using desktop software and the returns must be mailed in.
If you need to prepare a return for 2016, 2017, or 2018 you can purchase and download desktop software to do it, then print, sign, and mail the return(s)
Remember to prepare your state return as well—if you live in a state that has a state income tax.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.
Note: The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac. It cannot be used on a mobile device.
If you are getting a refund, there is not a penalty for filing past the deadline. If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.
The TurboTax online editions are only for tax year 2019.
Clear the tax return since you entered 2018 tax data.
If you have not paid for the online edition you are using, have not filed your tax return or registered the Free edition, then you can clear your return. Click on Tax Tools on the left side of the program screen while working on the 2019 online tax return. Click on Clear & Start Over.
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.
Or If you have a simple return You can fill out the blank forms for free.
Here is the IRS 2018 Form 1040
And instructions https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf
2018 Tax and EIC Tables https://www.irs.gov/pub/irs-prior/i1040tt--2018.pdf
IRS 2018 Schedules 1 thru 6 (only if needed)
Don’t forget your state. And you will have to print and mail your returns. So be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.
By the way, how did you do 2019? Did you use this same account or start a new account when you entered 2018? If you tried to do 2018 in the same account you just overwrote your real 2019 return so you can never get a copy of it.