@mnhemenway said "Yes, my earned income was more than half of my support. I already made that clear."
No, you only said you supported yourself, but not how.
So, if you are not a dependent, and you supported yourself with earned income, you ARE eligible for the refundable portion of the AOTC. It's the IRS that's wrong. Being under 24 does not, alone, disqualify you. You should respond to them with proof that you supported yourself with earned income.