The number of classes cannot be fewer than 1. 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 statistical groupings for a specified number of classes (an unsupervised classification). If you specify that the number of classes is fewer than one, you are telling ArcGIS to classify all cell values into no classes.
Specify a positive number of classes.