Description
In order to discriminate among features for clustering purposes, the Analysis Fields you select should contain a range of numeric values. Any fields where 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 you specified have variation.
Solution
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.