Description
The configuration of the Input Features and/or the parameters defining which features can be a part of the same cluster make it impossible to assign every feature to a cluster.
Solution
Try creating a spatial weights matrix using the K nearest neighbors option and increasing the Number of Neighbors until all features can be clustered. Experiment with different Number of Neighbors values to see how this impacts your results.
Alternatively, run the Multivariate Clustering tool which does not impose any Spatial Constraints and include Analysis Fields that reflect the spatial relationships you want to incorporate into your analysis.