IRS Letter 12C is normally sent to reconcile payment owed on a premium health care plan. If you did not have health care and filed that you had healthcare with the marketplace, then the IRS will normally contact you and resolve this without having to file an amend.
Your best option would be to call the IRS and see what needs to be done to resolve your issue.
Here is the number and the quickest way to get to talk to a person:
Call them at 1-800-829-1040. They are open Mon - Fri from 7:00 AM - 7:00 PM. The best time to reach them is 8:00 AM your time.
Get to a live person fastest: Press 1 for English, then press 2 for Personal Income Taxes, and then push 1, then 3, then 2, then 0# when it asks for your social security number, then 2.
