The Sorry Server and Not Available Redirect are mutually exclusive options. Both are ways of handling situations which would otherwise result in a downed Virtual Service.
The Sorry Server sends all traffic to that server without checking its health. Not Available Redirect instead replies to incoming requests with an error code and redirect URL or error message.
These are two options for handling the same situation, but they cannot be used in conjunction.
To enable the Not Available Redirection Handling clear the "Sorry" Server address and port then click Set Server Address. When the page refreshes, Not Available Redirection Handling will be directly below it.
If you are still having trouble locating this setting, please confirm that the service type is set to HTTP/HTTPS. Other types of service are unable to perform redirection because it relies on specific HTTP functionality.