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...

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.