What is the Payer's TIN number for IRS? Since IRS processed my return late, they send me interest on 2020 return and asked to report it in 2021 tax as income. Thanks!

Just report as interest income. You don’t need a tax ID 

Form 1099-INT doesn't require a Payer's TIN when you enter your IRS interest. Delete 00-0000000 and resubmit your return. 

I’m grateful for your comments. I submitted without the Payer’s ID and it went through. Much appreciated!