IRS Notice CP27 informs you that you may be able to claim the Earned Income Credit to reduce your taxes or get a refund.
Respond to Notice CP27
First, review your Form 1040 as filed with the IRS. If you don't have it, you can order a Tax Return Transcript by calling the IRS at 1-800-829-1040 or use the website.
But, to actually make the claim for your refund you will have to complete a Form 1040X.