How to activate Apple Music subscription purchased through MTN SIM card?

You need to contact MTM since that is who you are paying for the subscription. They need to link your account and that has not been done. That is a very common issue where many users have asked the question here and contacting MTM resolved the problem.

You should receive an email with a url to initiate the link process.

click here ➜ Connect Apple subscriptions from your wireless carrier or other provider to your Apple Account - Apple Support

and here ➜ https://play.mtn.ng/apple-music