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.
Your 2018 AGI is the way the IRS verifies that the same person who used the Social Security number you are using is filing the 2019 return. If you cannot use the 2018 AGI that matches the IRS database, you will not be able to e-file. If you cannot find it your other option is to try using "zero" as your 2018 AGI. If zero does not work then you will have to print, sign and mail your 2019 tax return.