Hi there!
You can try signing in yout Apple ID here https://tv.apple.com/ and check you will see the offer to get the 3 month free trial. You might also get a promo code that you can activate on the website from the retailer. At this point, I'm afraid they're no longer offering the 1 year trial though.
Also, please note that Offer must be claimed in the Apple TV app within 3 months after first setting up your new device.
If you still can't redeem the promo, please see this article:
If you can't redeem your promotion for an Apple subscription - Apple Support
If you need further help, just Contact - Official Apple Support
Hope that helps! Best of luck!