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 Polygons For Aggregating Incidents Into Counts, the tool counts the number of incidents within each polygon and uses the count values for the analysis field. In your case, the count for each aggregation polygon is exactly the same (there is no variation at all in the count values).
Solution
You can provide different Polygons For Aggregating Incidents Into Counts, try a different Incident Aggregation Method, or select an attribute in the Input Features to serve as the Analysis Field.