Book that one as if you are not on the dining plan (don't enter your resort reservation), because they don't accept it. If the system thinks you are on the dining plan it will not let you book restaurants that aren't on the dining plan. That's not really logical but that appears to be how it is set up.
If you can't book it through Disney, call RFC directly. It's not a Disney restaurant and not all their tables are available for booking through Disney.