"last year"'s return would be your 2023 return, since right now everyone is working on their 2024 returns. Are you trying to find your 2023 AGI?
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 2022 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.