The first and second stimulus payments were tax credit advances on a 2020 tax return. The second stimulus payment received in 2021 is entered on the 2020 federal tax return. As shown on the stimulus payment page in the Federal Review section of the TurboTax program.
You aren't claiming it as income, so it doesn't matter that the payments were received in different years.
You are reconciling the credit - and to do so the program needs to know if you already received both payments, and if so the amount in order to determine if any additional amounts are due to you.
If you received it, and you indicate that you didn't, then the program will generate an additional credit for you. That credit will not match the IRS records and your entire refund will be delayed while your return goes through review and adjustment.
I know we have all been conditions to what year income belongs to, but as other have posted, these stimulus payments are not income so what year you actually received it doesn't matter...in fact these are PAYMENTS... if you are due more, it ends up on Line 30 and if you look closely, Line 30 is part of the PAYMENTS section of the income tax return.