HBO announced early last month that it would retire its HBO Now (and HBO Go) app on second- and third-generation Apple TV set-top boxes at the end of April.
As of this moment, the HBO Now app is no longer available on those older Apple TV units.
If you use these set-top boxes to watch HBO shows through the Now app, you might want to switch to other hardware such as a newer Apple TV, a Roku, or Amazon Fire TV.
Alternatively, you can subscribe to HBO Now through various providers such as Apple TV Channels.
It's important to note that HBO Go is still available on those older Apple TV models, but not for long as the app will soon leave those devices, as previously announced by HBO.