Trying to add a secondary axis on a line chart with two data series. I've followed the instructions on the Microsoft Support web page here:
https://support.microsoft.com/en-us/office/add-or-remove-a-secondary-axis-in-a-chart-in-excel-91da1e2f-5db1-41e9-8908-e1a2e14dd5a9#articleFooterSupportBridge=communityBridge
"In the chart, select the data series that you want to plot on a secondary axis, and then click Chart Design tab on the ribbon. For example, in a line chart, click one of the lines in the chart, and all the data marker of that data series become selected. Click Add Chart Element > Axes > and select between Secondary Horizontal or Second Vertical."
However, when I select Add Element - Axes, no option for a secondary axis appears, only 'primary horizontal and vertical'

What am I doing wrong? Thx