Do you mean you have some 2020 income you did not put on a 2020 tax return? NO you cannot put it on a 2021 tax return. If you did not file a 2020 tax return then you can still do that but you need to use the correct 2020 software and 2020 tax forms. Or if you filed a 2020 return and forgot to enter something, you will need to amend the 2020 return you filed to add that income.
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