I subscribed to Starz thrugh Apple tv. It works great there, but I can't get it on the apple app on my mac.

Be sure to sign in to the TV app (menu Account) with the Apple ID account that has the subscription.

Subscriptions to Apple TV app Channels should then be available to you.

Subscriptions started outside the Apple TV app (e.g. in the Starz app on the Apple TV box) would not be supported in the Apple TV app on your devices.

The Apple TV app, as a real app, is available with macOS 10.15 or later, not 10.14 or earlier.