Why I am being charged for Apple TV subscription during free trail?

Please verify what you were charged for exactly.

View your purchase history for the App Store and other Apple media services - Apple Support

If you canceled the Apple TV+ subscription during the trial period, then the subscription would expire immediately. Trying to access Apple TV+ content after that may prompt to resubscribe at full subscription rate.

If you tried to access non-Apple TV+ content, either from the Store (buy/​rent) or from a different in-app subscription service, then you would be charged for that separately.

How long do you think your free trial lasts? If you’ve been charged since the beginning it means you did not properly apply for the free trial. You may have assumed you did but you did not.

If you can't redeem your promotion for an Apple subscription - Apple Support