I am trying to add a secondary axis to my PivotChart (in the same
manner as a normal Chart) and find the option is greyed out:

I select the data series,
Go to Format Selected Data Series Axis tab
The "Plot Series on" radio button selector is greyed out.

Is there a way to activate this option for PivotCharts? Or some other
way to achieve a similar result?

I just want two Y axes on either side of the report with different
scales to appear.