The easiest way to find your 2017 AGI is to visit the IRS website linked below to obtain a transcript of your 2017 tax return. You'll find your 2017 AGI on this transcript.
https://www.irs.gov/individuals/get-transcript
Once you've found the correct AGI as per the IRS, you can follow the instructions below to correct the wrong AGI and complete your e-filing.
https://ttlc.intuit.com/community/agi/help/where-do-i-correct-my-agi-in-turbotax-online/01/26311
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If your 2017 return was filed late, or if you amended it, then the AGI in the system might not match, or might not even be in the system. Try using zero for your AGI instead. If zero does not work, then you need to print, sign and mail your 2018 return.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.