I received a letter 6470 from the IRS, regarding my 2020 return. I do not understand this letter. Can anyone explain?

Go to this IRS website for information on the 6470 letter - https://www.irs.gov/individuals/understanding-your-letter-6470

It explains your right to appeal the IRS decision concerning the Recovery Rebate Credit on the 2020 federal tax return.

Apparently you also received a notice from the IRS notifying you that they made changes to your federal tax refund because of the Recovery Rebate Credit you entered on your 2020 federal tax return.  If you indicated on your return that you did not receive either or both stimulus payments the IRS probably denied the credit and removed it from the federal tax refund.

Look at your tax account and see if the stimulus payment are shown as being sent to you and if you are married filing jointly look at the tax account for your spouse also - https://www.irs.gov/payments/your-online-account