Description
The configuration of the input features and/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 Spatial Constraint and increasing the Number of Neighbors until all features can be grouped. Experiment with different Number of Neighbors to see how this impacts your results.
Alternatively, run the analysis without imposing any Spatial Constraints (NO SPATIAL CONSTRAINTS option), but rather include Analysis Fields that reflect the spatial relationships you want to incorporate into your analysis.