On what device is this happening?
I can't watch a movie despite having a valid Apple TV + Subscription
Not all mentioned content in the Apple TV app is part of Apple TV+. The app can be used to access (much) more, e.g. buy or rent movies, and those are not included in the Apple TV+ catalog, instead available on a per title purchase basis.
I'm trying to watch a movie called "The Whale" which I believe is included in the subscription.
Unfortunately for you, this title is not included in Apple TV+ (Apple Originals), and is instead offered from A24 Studios with Apple as reseller.
… it only has options "how to watch" and "add to up next" but I cannot actually view it.
It seems to me like you are trying to access this on Fire TV, Android TV, or Google TV smart platforms, including Sony TV. The “How to Watch” blurb was supposed to convey that users should do the buy or rent or extra subscribe option on a different supported device (e.g. iPhone). Then after the financial transaction has been done, it should be available on your main Apple TV app device as well (through the matching Apple ID account) in the app’s Library. Unfortunately, at this time, the Apple TV app doesn’t support additional Store transactions on these mentioned devices.
Get the Apple TV app or Apple TV+ app on your smart TV or streaming device - Apple Support
“2. In the US, you can subscribe to Apple TV+ on Amazon Fire TV, but you can't purchase other content from the Apple TV app. On Amazon Fire TV (in other regions), Android TV, and Sony TV devices, you can't purchase content from the Apple TV app. You can buy, rent, or subscribe on another supported device and then watch in the Apple TV app on your Amazon Fire TV, Android TV, or Sony TV device.”
