you need your 2018 adjusted gross income (line 7) from 2018. you don't say why you don't have a copy, thereefore i can only offer you this suggestion. register at IRS website and get a transcript of your 2018 return
https://www.irs.gov/individuals/get-transcript
if you can't get it you'll have to paper file (also state return, if required will have to be paper filed)
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