Have you filed yet? If you have not filed then go to the Stimulus section and enter the correct amounts you already actually got in each box.
When you go through the federal review tab it will ask how much you got if any. Enter a 0 in the box if you didn't get one. Or to enter your stimulus payment in TurboTax, type "stimulus" (without the quotes) in the Search box, then click the link that says "Jump to stimulus."
After you have entered your tax information and are ready for the Federal Review, it will be the first question.
If you are trying to trigger it, you can go to Other Situations, answer any questions and click Done. The next automatic selection is Federal Review and it will ask about payments received.
I filed my 2020 tax return, and adjust gross income is showing -$584,408, and Schedule C deductible mortgage interest is showing $7,761,096. The computer calculations? Do need to do any amendment?