I often get the error: unable to calculate route. This happens for instance when I select a railway station (a white dot) as a destination. Since you’re not able to drive up to the platform this is actually quite logical. My solution in these cases is to drag the destination from the platform to the nearest road.
On another occasion I choose a destination (52º21’53.6″N 4º57’13.6″E) not reachable by car, but when switching to cycling mode the route calculation worked fine.
It would be nice if PE snaps to the nearest road automatically, using a reasonable margin off course.