I updated the question title to include the name of the study: VolumeAvg. In the screenshot below you will see how we begin to build this code using the Condition Wizard. Be sure to check the following video for details on how to use the Condition Wizard to build custom scans.
You will see in the screenshot below that the VolumeAvg study contains two plots. You chose the plot named "Vol". So you were basically checking if the current volume is greater than the current volume times 2. Which will never be true. Because current volume is always equal to current volume. To get this to work you need to select the other plot from the VolumeAvg study which is named "VolAvg".
Once we make this adjustment we can add the "times 2" to the code and we have this:
plot scan = volume is greater than VolumeAvg()."VolAvg" * 2;
This works as expected.