Migraine is a disease that most often causes moderate to severe headache on one side of the head. A migraine attack is a headache that may be accompanied by throbbing, nausea, vomiting, sensitivity to light and sound, or other symptoms. The goals of the study are to evaluate adverse events and how well treatment of atogepant works compared to placebo (looks like the study treatment but contains no medicine) in preventing chronic migraine in participants between 12 and 17 years of age. Atogepant is a medicine currently approved in the United States and Europe for the preventive treatment of migraine in adult patients with migraine and is being studied for the preventative treatment of chronic migraine in participants between the ages of 12 and 17 years. Participants will be randomly assigned to one of the 2 groups to be treated with either atogepant or placebo. This study is double-blinded, which means that neither the patients nor the study doctors know who is given which study treatment. Approximately 420 participants 12 to 17 years of age with chronic migraine will be enrolled at approximately 70 sites across the world. Participants will receive oral tablets of atogepant or placebo once daily for 12 weeks and will be followed for 4 weeks. Participants will attend regular visits during the study at a hospital or clinic and the effects of treatment will be checked by completion of a daily diary, medical assessments, blood tests, checking for side effects, and completing questionnaires.