How do I amend line 19 to enter an amount for 2nd stimulus payment

Have you filed yet?  Is the amount on 1040 line 30 wrong for the Recovery rebate credit?  If you have filed the IRS will automatically adjust your return for it.

If you have not filed yet.........

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.

If you didn't get one or both of the Stimulus payments or qualify for more it will be the Recovery rebate credit on 1040 line 30.  It either adds to your refund or reduces a tax due.