Free 12 months Apple TV subscription not applied to account when MacAir purchased

The offer is for Apple TV+, not Apple TV, which is different. The offer is not automatically applied. The user has to redeem the offer to activate it and start the subscription. There are a few conditions, like you have to redeem within the first three months, you can redeem only once, etc. To redeem on Macs, remember to first sign in to the TV app (menu Account) on your new eligible device, to even see the redeem button. The offer is for Apple TV+ content specifically. Other content in the Apple TV app is not included, and will have to be payed for separately. The offer is for one account, with which the new device was originally activated. Other accounts wouldn’t get this offer. Read here if you have trouble redeeming: If you can't redeem your promotional trial of Apple TV+ - Apple Support.

I have contacted Apple support twice about this.

That should have resolved any issues. None here can change what they decided differently.

• See your subscriptions overview - Apple. Verify that Apple TV+ is listed as an active subscription, and what its next billing date is (12 months from the start date?), and if that matches the Subscription Confirmation email details.
• See your purchase history for the App Store, iTunes Store, and more - Apple Support. Verify that the charge is for this subscription, and not for something else.
• If the charge is not listed in your purchase history, then the invoice is for a different account, or the message is not even legitimate.

The offer is not automatic, it needs to be claimed within 3 months of device activation (steps below). After that has expired it will revert to the standard trial of 7 days. As of July 1st it is for 3 months, however if you purchased before then and are still within the deadline for the offer you will be eligible for the initial 12.

If you received an invoice from Apple it could also be for something else billed to your account. You can check your account to verify the status of your subscription and also purchase history.

1. Turn on your new Apple device and sign in with your Apple ID.
2. Open the Apple TV app. Make sure your device is running the latest version of iOS, iPadOS, tvOS, or macOS.
3. The offer should be presented immediately after launching the app. If not, go to the  Apple TV+ tabwhere you'll see the offer.
4. Tap Enjoy 12 Months Free.