My experience is with a Vista HCx, but I expect the same applies...
This error usually means you don't have a routable map selected. Either you've downloaded the map without the routing data (there's a tickybox in Mapsource iirc), or you've overlayed it with a non-routable (topo, openstreetmap, etc) map.
Top tip: in the "setup map" menu's "information" tab, where you can select/deselect individual maps by clicking on their respective tickyboxes, press the menu button again. You'll be given the option to show/hide all maps of a given type in a single click. This means you can switch between routable maps like City Navigator or the basemap and non-routable but sometimes more useful Topo maps in a couple of clicks, rather than spending the next three million years scrolling through the entire UK's mapping in search of the one you care (or specifically don't care) about.
Annoyingly, TopoGB is always rendered over City Navigator, which nobbles its routing capability. As a compromise, I usually use the
SMC Contour Maps in combination with the City Navigator maps, for a routable map with visible contours.
