Why do you think you need to add AGI to a 2023 return? You only needed to add AGi to a 2023 return in order to e-file it ---back in 2024. It is too late to e-file a 2023 return, so AGI no longer matters for a 2023 return. The only way to file a 2023 return is to file it by mail. You have to print it, sign and date it ink, and mail it in.
If you are trying to e-file a 2024 return, then you are being asked to enter your 2023 AGI from your 2023 tax return.
Where do I find my AGI?
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947
Your 2023 tax return shows your 2023 Adjusted Gross Income (AGI) If you filed a joint return then the AGI is the same for each of you.
Form 1040 line 11
If entering the amount from your 2023 tax return does not work, then try entering “0” instead. If that does not work and you still cannot e-file, then print, sign, and mail your tax return.
You can get a free transcript of your past return from the IRS:
https://www.irs.gov/individuals/get-transcript
Note: If you filed late or amended your return, your correct 2023 AGI may not be in the system.
https://ttlc.intuit.com/questions/1944348-how-do-i-print-and-mail-my-return-in-turbotax-online
Note: If you mail your federal return, you will also need to mail your state return. You will not be able to e-file your state return.