Interestingly, if one of the data series is shifted to the right y-axis, it is possible to edit the y-axis title on the right, while it is still not possible to edit the left y-axis title.
I saw a similar question in the community but it was unanswered.
I found out the root cause. Once there are more than one data series in the y-axis, QS would disable the option for user to add a y-axis title for all of the data series.