"A ti no te interesa la música."
Translation:You are not interested in music.
John, No te interesa la música = You are not interested in music (Music is not interesting to you.) (Music doesn't interest you.)
The te is required. It's the indirect object pronoun that this type of verb uses (gustar, encantar, interesar, importar, etc.).
The a ti is optional. Te doesn't need a clarifier; it's as clear as can be. But in some situations it might need to be emphasized (perhaps during an argument), and that's why it's used here.
Since there's nothing in the context of the sentence that requires emphasis on you, Duo should accept the answer without the a ti.
You might like studyspanish.com (Grammar Unit Four: object pronouns and verbs like gustar).