I heard back from many users. The most common response was to place 0 in prior year AGI. I did that and it worked. My guess is that since the checks and the returns are separated upon receipt at the IRS, the return either got lost or is sitting in a big pile waiting to be processed. Since I paid my balance due and it was recorded at both the IRS and by my bank, I will await correspondence from them if they need a copy of my 2019 return.
Others also posted that they sent off another copy with an explanation letter, but no one said that the second copy was received or processed. I think that furthers the belief that they just have not processed a huge number of returns.
If the zero does not work for prior year AGI, someone else tried checking that no return was filed in prior year and that worked for them. Good luck!
