If the AGI from your non-filer return was $1, then you can attempt to efile with that. If it is rejected, then use zero for your 2019 AGI. In your case it is also OK to mark the did not file in 2019 if that is the only way you can efile for 2020. The IRS should have your 2019 AGI on their database to check your return. But AGI is just a security check and not having the correct AGI for efile will not affect your tax return.
