See the following FAQ and video:
FAQ: How do I find last year's AGI?
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/00/25947
TIPS:
Use the 2018 AGI from your original 2018 return, not an amended one.
If the return was filed real late in the year, such as November 2019 or later, use an AGI of 0 (zero).
If it's a married joint return, both spouses use the same total AGI. Do not split it or otherwise allocate it.
If you qualify to open an IRS account, you could download a 2018 tax return transcript at the IRS website, to see what they show for your 2018 AGI.
https://www.irs.gov/individuals/get-transcript
FAQ: What if I entered the correct AGI and I'm still getting an efile reject?
https://ttlc.intuit.com/community/rejections/help/what-if-i-entered-the-correct-agi-and-i-m-still-getting-an-e-file-reject/01/27031