Hi Wendy,
The histogram necessary for a Classified raster is produced by Calculating Statistics. I'm not sure why the statistics (and the histogram) weren't generating in ArcGIS Pro from the Symbology pane when prompted, but Calculate Statistics from the Catalog pane is always a good next step if that happens.
If you're interested in the wider scope of the question, raster statistics tell the program how to symbolize each cell. Minimum, maximum, mean, and standard deviation, etc, are necessary for stretch symbology and for classified symbology, since without them, it's not clear to program what each cell's value should look like.
If you'd like to read more, here's some of the documentation on raster statistics:
Raster data statistics—Help | ArcGIS Desktop (though this is ArcMap documentation, it's still applicable to ArcGIS Pro)
Raster statistics (Environment setting)—Geoprocessing | ArcGIS Desktop
Calculate Statistics—Data Management toolbox | ArcGIS Desktop
Happy rastering!
-Lauren