A Free Trial can only be used once, so if you attempt to use multiple free trials, you will be charged immediately. The offer most likely said that New Subscribers were eligible for a free trial. The same is true for Apple Music, Apple Arcade, and even Apple TV. You were not being targeting as an eligible person, it is the same offer available for all services. The same is true for the Free Trial of Apple Music with the purchase of AirPods. The trial is only for new subscribers and if you already used a trial, you are not eligible. You will see in this example it specifically states New Subscribers.
You can try to request a refund here:
Request a refund for apps or content that you bought from Apple - Apple Support