This tool was designed to work with an analysis field that contains a variety of values. If the values for the analysis or weight field are all 1, for example, there is no variation in the values and the computed variance will be zero. When the variance is zero, results cannot be computed.
Choose an analysis or weight field with a variety of values. If you are working with incident data where the weights might all be 1, consider aggregating the incidents before running the analysis so that the weight is the number of incidents that fall within each aggregation unit. Consider using a tool that does not consider weights but only considers feature locations, such as the Average Nearest Neighbor and Multi-Distance Spatial Cluster Analysis (K Function) tools.