Did you get married in 2020 and are filing a Joint return? You need to enter both of your Single AGI to efile.
You need your 2019 AGI to efile 2020. The AGI is on 2019 1040 line 8b.
How to find the AGI.
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947
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
https://www.irs.gov/individuals/get-transcript
