What's the version and build number of your VCSA?
I guess it's because of incomplete process of developing and embedding vSphere full features in to the HTML5-based web client. And in older versions you couldn't find some advanced features but as you said FLEX is ok. I had similar issues like that before especially in earlier release of vCenter 6.7Please mark my comment as the Correct Answer if this solution resolved your problem
Which vCenter version (and update) do you have?
Have you tried in html5 using another type of browser?
Yes, tested on multiple browsers.