The IRS states:
"If you received a letter from the IRS about the Earned Income Tax Credit (EITC), also called EIC, the Child Tax Credit/Additional Child Tax Credit (CTC/ACTC) or the American Opportunity Tax Credit (AOTC), don't ignore the letter (notice). Follow the directions on your letter. All the information you need is in your letter. This page explains your notice and gives tips on what you need to do now."
Please click I Received a Letter from IRS about My Credit; What Should I Do? to learn more.