Check these links for tips:
"Get 3 months of Apple TV+ for free." - Click on the link and read it, including the FAQ at the end. Check your eligibility. --> https://offers.appletvapp.apple/
"Only one offer per Apple ID and only one offer per family if you're part of a Family Sharing group, regardless of the number of devices you or your family purchases. "
'If you can't redeem your promotional trial of Apple TV+' click here --> https://support.apple.com/HT211767
You must sign in on your new device. If you have problems, contact Apple. They can provide a code for you to redeem.
Get Apple TV+ free when you buy a new iPhone, iPad,
iPod touch, Apple TV, or Mac. - Terms and conditions
US - https://www.apple.com/promo/pdf/EN_US_ATV+_Promo_TandCs.pdf
UK - https://www.apple.com/uk/promo/pdf/UK_TV_Plus_Promo.pdf
Latin America - https://www.apple.com/la/promo/pdf/ES_LAMR_ATV+_Promo_TandCs.pdf
Contact Apple for support and service --> "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232