Thanks for the replies. I did manage to get someone on the phone with the IRS (took about 2 hours to get to the right person). It wasn't about a carryback claim (as per the notice I received). It was about a deferred self-employment tax amount from 2020 (part of Covid relief). I had paid half of it in 2021, but the IRS marked my tax year 2020 as paid in full and never sent another bill for the remaining half. Admittedly, I didn't notice that I never paid for the second half. So, I did owe this amount that their computer systems discovered, but the notice that they sent was totally wrong about why I still owed them money. 😑
