Where on the 1040 do i claim. An unpaid stimulus check?

You can use the recovery rebate credit in Federal Review to get the 3rd EIP payment.   The 3rd one is the ONLY one you can get on a 2021 return.   It will show up on line 30 of your tax return.

If you are eligible but did not receive your third stimulus payment, you can claim it on your 2021 tax return as the Recovery Rebate Credit.

In TurboTax Online, to claim the Recovery Rebate credit please do the following: 

1. Sign into your account and continue from where you left off 
2. Click on Federal in the left-hand column, then on Federal Review on the top of the screen
3. On the next page titled Did you [or your spouse] get a third stimulus payment?, answer Yes or No
4. Follow the interview and answer all questions
5. TurboTax will determine whether you are entitled to any additional stimulus
6. Any stimulus amount remaining due to you will show as a credit on line 30 of your form 1040.