There is no need to amend the 2021 tax return because the IRS changed your tax return.
The IRS changed your return because you indicated that you did not receive the 3rd stimulus payment but the IRS has records which show that the stimulus payment was sent to you.
Go to this IRS website for your tax account to see if the 3rd stimulus payment is entered - https://www.irs.gov/payments/your-online-account
If you checked your bank account in the March - May 2021 timeframe and there is not a EIP3 payment from the IRS then go to this IRS website for how to start a trace on the payment - https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
