I can't get my Apple TV to cast on my Samsung smart TV

Verify that your smart TV supports AirPlay, as your MacBook Pro does not support competing local streaming ‘casting’ protocols. The AirPlay button can be found in the controller bar of the player window in the TV app.

List of AirPlay 2-enabled smart TVs - Apple

If your Samsung smart TV isn’t too old then it may also support the Apple TV app directly, and then you wouldn’t need AirPlay for Apple TV+ content.

List of Apple TV app-supported smart TVs and third party streaming devices - Apple

2016 Samsung Smart TV

Search Samsung’s app repository for apps from Apple.

Besides the Apple TV app for newer and higher end TVs, there is also the Apple TV+ app, which brings just the Apple TV+ content (i.e. a subset of the former). I haven’t seen a list of supported models for that, but it should be available on slightly older models that do not support the Apple TV app. Cutoff might be around 2016 (Tizen 2.4?).

Get the Apple TV app or Apple TV+ app on your smart TV, streaming device, or game console - Apple Support

Use apps on your Samsung smart TV and projector - Samsung Support