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.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
