I entered my AGI from line 8b 1040 - 2019, rejected. IRS transcript for 2019 says my 2019 tax return was never processed. Enter zero in my AGI to submit electronically?

Yes - try zero.

I submitted with Zero for AGI and it was accepted.

It worked, makes me nervous that last years taxes have yet to be processed 🙂

It does not mean 2019 was not processed, just that it was process too late to be included in the 2020 e-file database.

Did you receive your 2019 refund or was the tax paid cashed or deduced from your account?