Description
To discriminate among features for clustering purposes, the Analysis Fields values you select must contain a range of numeric values. Any fields in which all of the values are the same (all ones, for example) will be dropped from the analysis. In this case, none of the Analysis Fields values you specified have variation.
Solution
If there is an active selection set in which all of the selected features have the same values for Analysis Fields, change the selection to include features with more variation in the Analysis Fields values. You can also include additional variables that will make it more effective at discriminating among features.