The IRS recommends entering a 0 for the 2019 AGI - https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue
E-filing your 2020 tax return: To e-file you will need to enter your AGI from your tax year 2019 tax return. If your 2019 return has not yet been processed, you may enter $ 0 (zero) as your prior year Adjusted Gross Income.
