Hi Chenyi- Julianne from Mixpanel Support here!
Not all metrics support 'View Users', which is likely the limitation you're running into here. For example, you won't be able to view users or events if graphing a cohort over time.
I definitely recommend submitting your use case and request for the feature via the 'Submit Feedback' form (under the ? menu) with details about the specific metric you're using. This will flag your use case and support of the feature to the Product team.
