How do I enter my spouse's AGI if filing separately?

Your 2019 AGI should still be used to file your return.  If that does not work, there can be several reasons why.

If a return is amended, then the AGI is set back to zero.  This year the IRS is very behind on processing returns due to COVID-19.  So they may not have the same number on their database that TurboTax does.

There are two ways to work around this.

First try to file with zero in the 2019 AGI field.

If that is rejected then file again but select the option that you did not file last year.  The IRS does not see that question and it will allow you to efile with no AGI.