When performing a classification, generally you are classifying a series of raster bands into classes. The classes are defined by the statistical properties of the identified classes. The classes can be identified by specified polygons (a supervised classification) or by a statistical grouping for a specified number of classes (an unsupervised classification).
If you specify fewer than two classes, you are telling ArcGIS to classify all cell values into one class. If this is the desired result, do not use the multivariate tools; instead, use the reclass tools.
Make sure you specify more than one class.