Why is this block found but not installed?
In general, that can have many possible causes. But it's very likely some sort of syntax or other error that leads to an exception or a fatal error.

Open the Place block link in a new tab to see the response, check the network tab and/or check your apache or drupal logs.

That should give you a hint about the problem. In this specific example, it was a trailing space after the closing PHP tag, as suggested by @kiamlaluno, that's why Drupal coding standard says that they should be left out.