You can not change or fix your AGI that was reported on your 2018 tax return.
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are you saying that the 2018 agi used to e-file your 2019 return caused a rejection? if not, clarify. it should be the amount on line 7 of your 2018 return. if you didn't file or filed lat, try $0. if that doesn't work ou can get a transcript of your 2018 return from the IRS website which will show your AGI.