The reduction in payroll taxes (that's how ERC affects the Profit and Loss side of the financials) will increase your net income. Net income, in turn, should increase your QBI deduction (it's not a credit, but rather a deduction). What amount do you see on line 13 of your originally filed Form 1040? And what number do you see on that line once you amend the numbers to reflect the reduced payroll taxes?
There is a possibility that with the increased income you are hitting the threshold for SSBT businesses that phases out the QBI deduction. Read more about it here.
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