There was an error when adding raster layers to the stack. The Class Probability and Principal Components tools each create multiband rasters outputs to store their results. In the workspace Info directory, the tools first create the specified stack name. The stack is then populated with the results as individual rasters (called layers), naming them by appending the layer number to the stack name. For example, if the output stack name is mva1, the first raster layer will be named mva1c1, the second mva1c2, and so on. This error will occur if one of the default band or layer names already exists in the workspace.
Check to make sure that there are no rasters in the output location that have a name of the intended output stack name followed by a letter c and a number.