Hi Ian, OK I understand. RealPlan is pretty slick in the majority of cases, but sometimes it chooses some "interesting" routes (or none at all, as in your case). Even when it finds a reasonable-looking solution, in some areas with mountains or large bodies of water, it chooses a route that a cautious pilot such as myself wouldn't fly.
My suggestion (and the suggestion in the error dialog box you took a snapshot of) is to add an intermediate waypoint(s) along your route that would "guide" RP towards the desired solution. Conversely, you could say, well, what's the point of RealPlan if I have to help it find a solution? I think RP probably works for some (a lot?) of people for at least a lot of the time, but...