Yes, if you didn't claim the credit on your original tax return, you will need to file an Amended U.S. Individual Income Tax Return, Form 1040-X. The IRS will not calculate the 2020 Recovery Rebate Credit for you if you did not enter any amount on your original tax return.
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2020 Recovery Rebate Credit — Topic G: Correcting issues ...
How to File an Amended Tax Return with the IRS - TurboTax ...
