When you use a By() option in a platform the result is a list of platforms.
So, you'll need to get to the parent display box of one of reports of one of the platforms using the <<Parent message.
dt=open("$SAMPLE_DATA\Variability Data\3 Factors Nested & Crossed.jmp");
vc = dt<<Variability Chart(
Y( :Y ),
X( :Part, :Instrument ),
Analysis Type( "Choose best analysis (EMS REML Bayesian)" ),
Std Dev Chart( 0 ),
By( :Operator )
);
show(vc);
TestReport= report(vc[1])<<Parent;
TestReport << Set page setup( margins( 0.5, 0.5, 0.5, 0.5 ), scale( 0.80 ), header( 0), footer( 0), portrait( 1 ));
TestReport << SavePDF("c:\temp\Test.pdf");
Be sure to look at the log window after running the example above. The show(vc) will demonstrate that vc has a list of Variability platforms.
-Jeff