You could not find your 2019 AGI so you mailed your 2020 return? Then you have already filed your 2020 return. E-filing it now would put two returns into the system, causing confusion and perhaps delaying your refund for months. It is a really bad idea. Sit tight and wait for the IRS to process the return you mailed. It will take awhile. You do not say how long ago you mailed it. It is taking the IRS several months to process mailed returns.
If I misunderstood your post and you are talking about trying to e-file a 2019 return, you cannot do that now at all. E-filing for 2019 has been closed since mid-October of 2020.
If you downloaded 2020 to mail BUT you have not mailed it yet you can switch to efile. Click on Add a State to let you back into your retun.
FYI - Even though you have 2019 AGI its common this year for it to reject. If the AGI rejects try entering 0 for the AGI. That's because last year the IRS took a long time to process 2019 returns.