@skycamper - assume it was a mistake
1) here are the eligibiltiy requirements. Based on what you stated, you did it correctly. There are no time limits on LLC so the fact that the AOTC eligibility was exhausted shouldn't have impacted the LLC eligibility.
https://www.irs.gov/credits-deductions/individuals/llc
2) The credit is 20% of Line 31 or $1579. And it can be used to reduce Line 18, but the result can not take Line 22 below zero.
Follow the instructions specifically on the letter.
You can not amend a 2019 tax return at this point and receive a refund. There is a 3 year window to claim a refund and for the 2019 tax year that window closed (by law) on July 15, 2023 (unless you requested an automatic refund back then in which case the deadline is Oct 16, 2023 - tomorrow).
However, if you amend and the result is you owe, then you owe. that obligation never expires.
If the letter arrived in August, 2022 why are you just now attempting to amend (and as I stated, it is too late).
