Particle track starts from a source point and migrates based on the direction and magnitude of the flow field. This message occurs when the track encounters a cell where water does not have any velocity—if the water is stagnant, for example. As a result, the track cannot migrate any farther.
Check the correctness of the input data to Darcy Flow or Darcy Velocity, from which the input direction and magnitude rasters are created. The zero velocity magnitude could be a result of a particularly flat groundwater table, or it could be the result of bad head, transmissivity, or aquifer data.