Some of the options for the Incident Aggregation Method require a minimum of 30 valid point or polygon features; others require a minimum of 60 valid point features. The table in How Optimized Hot Spot Analysis Works outlines these requirements.
This tool will honor a selection set. If the Input Features you specified does have more than the minimum number of required features, check to see if there is an active selection that may be limiting the number of features being analyzed; otherwise, if you have less than 60 but more than 30 incidents, you can select an option for the Incident Aggregation Method parameter that only requires 30 incidents. Another option would be to perform your own aggregation prior to analysis.