The third economic impact payment is not taxable. Before entering the amount you received, TurboTax may have assumed you had not yet received it and was adding the amount to your tax return. Once you told the system that you already received your economic impact payment, the credit was removed from your tax return.
Form 8812 is used if you are eligible for the Child Tax Credit. TurboTax asks some questions to see if you are eligible and if you received any advanced child tax credit payments. The advanced payments are also not taxable.
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