I don't know for sure, but I suspect it's because the IRS hasn't processed your 2019 return yet, so they don't have your 2019 AGI on record. This happened to me because I had to file a paper return for 2019. I read that the IRS was so far behind, they had not yet processed 2019 paper returns. When I entered 0 for 2019 AGI on the e-File, it worked.
