If you can't find it you will have to print and mail your return.
You need your 2018 AGI to efile 2019. The AGI is on 2018 1040 line 7. If you filed a Joint return you use the same AGI for each spouse.
How to find the AGI.
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947