- Did you sign into the Apple Music app with your Apple ID/Password? If you are currently signed in, try signing out, then back in.
- Did you tap the Home button on the bottom of the screen?
The eligibility requirements are here and also the page showing you the redemption steps:
Offer for eligible new subscribers redeeming on eligible devices. Other new subscribers receive one month free. Auto-renews at $10.99/mo after offer period until cancelled. Offer not available if you are eligible for a 3-months-free Apple Music offer
https://offers.applemusic.apple/offer
If you are still unable to receive the offer, then contact Apple here:
Contact Apple for support and service - Apple Support