What exactly is mdlRF? Is it a "full regression model object" or a "compact regression model object" as the documentation page states the first input must be? Did you use one of the functions in the "Returned By" column of the table in the section describing the Mdl input to create mdlRF?
plotPartialDependence is only defined for certain types of objects and I'm guessing you're passing in a different type.
