You have included Analysis Fields that have the same or similar values.
In order to discriminate among features for clustering purposes, the Analysis Fields you select should contain a range of numeric values. Because there was no variation, the tool was unable to create the number of clusters requested.
Check if there is an active selection set where all of the features selected have the same values for Analysis Fields. If so, change the selection to include features with more variation in Analysis Fields. You can also include additional variables that will be more effective at discriminating among features or remove any Analysis Fields that don't have any variation in values.