The IRS sends a Letter 12C to inform you that additional information is needed to process the already filed tax return.
This letter is frequently used to reconcile payments of the Advanced Premium Tax Credit.
If your Letter 12C indicates that Form 8962, Premium Tax Credit was missing and not attached to your return, you will need to send a completed Form 8962 to the IRS.
If you purchased health insurance from the marketplace and had advance payments of the premium tax credit paid directly to your insurance provider, you are required to reconcile these advance payments on Form 8962. You should use the Form 1095-A provided by the marketplace to fill out the Form 8962.
You do NOT need to amend your return, so there is no need to submit Form 1040-X, Amended Return to the IRS.