are you sure it's from the IRS? normally a carryback claim would reduce taxes. This is what makes it so specious to me in addition to you being unaware of the claim. a carryback claim would require you to file an amended return for 2020. the IRS does not do carrybacks automatically. 2021 NOLs can not be carried back. section 1256 losses from regulated futures contracts can be. allowable casualty losses can be claimed in the prior year but they're not really carrybacks. certain other items can be carried back.
you should go online and review the transcript of your 2020 return and compare it to a copy of the return you filed.
it would also help us if we know the form number on the notice. but that does not make it legit.
if you have difficulties understanding the transcript try calling the IRS 800-829-1040
if the above phone number is on the notice it still may not be legit because of the difficulty in actually getting through to the iRS. don't trust other numbers.
if you have difficulties go to this website to get info about IRS tax advocate.
https://www.taxpayeradvocate.irs.gov/
