To redeem the offer from Apple, do so on your new eligible Apple device in the TV app, not on a different device.
The offer has be to redeemed within the first 90 days after device activation.
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 bring a solution. They can assist.
Note that for Macs, users have to additionally sign in to the TV app (menu Account), for the redeem option to appear.