You can't use TurboTax Online.
You must have a copy of the original tax return you want to update.
The simplest way is to prepare your correct tax return (using TurboTax CD/Download for example, or other means) and print it.
Then lay the paper printouts of both returns (correct and original) side-by-side.
You now have all the information needed to prepare Form1040X by hand.
It's pretty straight-forward.
The blank form in fillable PDF format and its instructions are available at www.irs.gov.
Mail to the same address you would mail your original tax return.
You could use the online version by first recreating the original return exactly as filed. Then pay the fees and say to mail it and print it out so it think you filed it. Then when it matches every line you can amend. How to amend an Online return https://ttlc.intuit.com/replies/7276596
Or you can buy the Desktop CD/Download program here, https://turbotax.intuit.com/personal-taxes/cd-download/
You can recreate the original return using the Forms Mode. Then say to mail it. How to amend, https://ttlc.intuit.com/questions/4667229-how-do-i-amend-my-2018-return-in-the-turbotax-cd-download-software