I have a spatial analyst licence.
First, make sure you are opening the tool under the Spatial Statistics toolbox. There is a big-data processing tool by a similar name under the GeoAnalytics Desktop toolbox which does not allow Explanatory Training Rasters.
Then can you provide a screenshot of the tool? I can see the parameter in the circled location.
Thanks for the reply! I need to purchase the image analyst extension before I can see the raster option appear.