If you are e-filing on TurboTax Online, please see the TurboTax FAQ for instructions on where to enter your AGI: Where do I correct my 2020 AGI in TurboTax Online?
I'm not an expert.
If you are filing electronically, and you used turbotax to file your 2020 taxes, turbotax will likely fill that data in for you.
Otherwise, you may be asked to enter it when you file electronically.
If you are filing by mail, I don't think you need that data.
My advice is that if you are ready to file, try to file and see what happens.
Reminder: Be sure to keep a copy of your return for your files.
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If you are e-filing on TurboTax Online, please see the TurboTax FAQ for instructions on where to enter your AGI: Where do I correct my 2020 AGI in TurboTax Online?