You have a Percentage measure to add to your chart and you want this to display on a Secondary axis (because the primary axis already has a sum measure and the difference between the two measures is too far out of proportion).
First make sure you have already added your percentage measure to the chart. You should see the measure in the Values section of your field list. You may see your percentage value as a small horizontal line compared to the other bars in your chart, or perhaps not at all. It’s easier to just select the % measure within the legend, right click the measure name and select Format Data Series. Here you can set the axis to Secondary, to the size of the bar using the Gap width and adjust the format of the Secondary Axis as required.