My federal tax was rejected because my AGI does not match what the IRS has on record. What number do they have if this is my first time filing?

If you are a first-time filer over the age of 16, simply enter 0 (zero) as your prior-year income for signature purposes. 

From the IRS article If this is the first time that you've filed a federal tax return and you are single, here's what you need before you start:

Todd's advice is valid even if you're not a first time filer.  My 2020 fed return was rejected twice, even though the 2019 AGI I entered was consistent with the IRS transcript I obtained.  TT Tech Support encouraged me to simply enter "0" rather than an actual number, and the return was accepted by the IRS on the third try.