First, find out if you are an Apple Music subscriber:
Manage App Store purchases, subscriptions, settings, and restrictions on iPhone - Apple Support
Or to view your active subscriptions directly on your iPhone, use: Settings > Apple ID > Subscriptions
General billing and subscription info: Subscriptions and Billing - Official Apple Support
If you are subscribed but can't access, restart the device and try again.
If you are not subscribed, but think you should be, confirm you're using the same Apple ID for all, and confirm you're within the same Family Sharing Group if you're using that and the Apple Music Family Subscription or appropriate Apple One subscription.
If you should have access but do not, contact Apple Support. Use the Get Support link in the upper right of this webpage.