Description
The configuration of the input features or the parameters defining which features can be a part of the same group make it impossible to assign every feature to a group.
Solution
Try using the K nearest neighbors option for the Spatial Constraints parameter and increasing the Number of Neighbors parameter value until all features can be grouped. Experiment with different Number of Neighbors values to determine how this impacts the results.
Alternatively, run the analysis without imposing any spatial constraints (use the No spatial constraint option), and include an Analysis Fields parameter value that reflects the spatial relationships you want to incorporate into the analysis.