Description
The Gi* statistic used by this tool requires an analysis field with a variety of different values. When you provide incident point data and also specify a Polygons For Aggregating Incidents Into Counts value, the tool counts the number of incidents in each polygon and uses the count values for the analysis field. In this case, the count for each aggregation polygon is exactly the same (there is no variation in the count values).
Solution
Provide a different Polygons For Aggregating Incidents Into Counts value, try a different Incident Aggregation Method option, or select an attribute from the Input Features parameter to serve as the Analysis Field value.