https://ttlc.intuit.com/questions/1900840-how-do-i-find-last-year-s-agi
If you can't find it in TurboTax, you can download a transcript of the return from the IRS: https://www.irs.gov/individuals/get-transcript
Sometimes the correct AGI will not match the AGI in the IRS database. If that is your situation, try entering the number 0 in place of the AGI. That will often work.
If it doesn't work, you will have to print and mail your tax return.
