so why zero versus the AGI for the 2019
Because if the IRS has not processed your 2019 tax return yet *and* entered it in their system, the IRS computer doesn't have an AGI for 2019 *in their computer*. So you enter zero. Sometimes you have to go one step further and tell the program that you did not file a 2019 tax return and let the program enter zero for you.
Typically, the order is:
1) Enter your final AGI from the 2019 return (I say final, because if you amended the 2019 return, that can change your AGI)
2) If that doesn't work, enter the AGI before you amended the return.
3) If that doesn't work, enter zero for the 2019 AGI.
4) If that does't work, tell the program you did not file a 2019 return.
5) At this point if nothing above works, you have no choice but to print, sign and mail the return to the IRS.
The IRS computers only allow you 5 attempts to e-file the return. After the 5th failure you have to mail it - even if you do have the correct AGI after the 5th failure.
