Unless the IRS letter says otherwise, I would prepare an amended return to remove the HCTC. Then reply to the IRS with a letter explaining that you claimed the HCTC by mistake, that you do not qualify for the credit, and that you have attached an amended return removing the credit. Do not file the amended return. Just attach it to your reply.
This is just my suggestion. I have no experience dealing with this type of problem.
As you may have heard, the IRS is severely backlogged this year. It will probably be 4 to 6 months before you get a response to your reply.
