If you didn't claim the credit on your original tax return you will need to file a Form 1040-X, Amended U.S. Individual Income Tax Return, to claim it.
The IRS will not calculate the 2021 Recovery Rebate Credit for you if you did not enter any amount on your original tax return, or you entered $0.
Use the Interactive Tax Assistant, Should I File an Amended Return?, to help determine if you should amend your original tax return.
For instructions see How to amend (change or correct) a return you've already filed.
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