It probably means that there is no path to the point you are trying to reach. There are some additional parameters for the function that let you
There are two optional paramters I need to add to the docs that give more control:
virtual bool Navigate(const dFloat x, const dFloat y, const dFloat z, const int maxsteps = 100, const float maxdistance = 1.0f);
However, the default values for these arguments usually provide good results. This suggests that you are trying to navigate to a position that cannot be reached because something is blocking it, so the closest point on the navmesh to your destination is greater than maxdistance.