I have an Apple TV subscription, but had to pay for a movie on the channel.

Movies that say rent or buy are from the iTunes Store and not free. 

Your subscription is for Apple TV + 

Apple TV + is Apple Original content. It’s a channel found within the Apple TV App. The shows included in your subscription will have the tv+ logo on them - those are free with your subscription 

 About Apple TV+ - Apple Support

Your subscription is for Apple TV+, one of the services in the Apple TV app.

The Apple TV app on third arty devices provides access to multiple services:

- iTunes Store movies and TV shows (Buy or Rent per item);

- Apple TV+ (note the differentiating Plus!) featuring Apple Originals (subscription);

- other Apple TV app premium Channels with third party content (subscription per Channel).