How do I activate my FREE Apple TV?

Here’s how to redeem the Apple TV +:

Turn on your new Apple device and sign in with your Apple ID.

Open the Apple TV app. Make sure your device is running the latest version of iOS, iPadOS, tvOS, or macOS.

The offer should be presented immediately after launching the app. If not, go to the Apple TV+ channel page where you’ll see the offer.

Tap Enjoy 1 Year Free.

https://support.apple.com/en-us/HT210573

Contact Support so they can verify your qualifying purchase and issue you a redemption code to use

I assume it was a purchase of a new quailing device (Mac, iPad, iPhone, Apple TV) from Apple or an authorized reseller. If refurbished it would need to be from Apple, as other sellers and open box items would not count. If that is the case and it still wasn't working, contact support as above.

Then, as explained contact support.