Please redeem this offer on your new eligible Apple device (iPhone), not on a different device. Once you have redeemed it, then the subscription will be available on any of your devices with the Apple TV app when signed in with the same Apple ID account.
The offer should be redeemed within the first 90 days. Else the offer expires.
Read here how to redeem: Apple TV+ offer - Apple
Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple
Read here if you have trouble redeeming: If you can't redeem your promotion for an Apple subscription - Apple Support
↳ Step 5 says to contact Apple if the prior steps didn’t resolve this.