As a consequence of this error message, the calculation usually stops. A flow case with free surface may require the calculation of direction of motion for nodal displacements on the free surface. If too much conditions are assigned, it is possible that the calculated direction of motion vanishes. The trouble typically appears when the direction of motion is zero or is tangential to the free surface. Make sure that the direction of motion do not vanish. In other words, the selected constraints, if any, should be compatible with the geometry.