You need your 2019 AGI to efile 2020. The AGI is on 2019 1040 line 8b. If you filed a Joint return you use the same AGI for each spouse.
How to find the AGI.
If you didn't file 2019 or filed it later in the year then try entering 0 for the AGI. If that doesn't work you will have to print and mail your return.
Or request a transcript from the IRS