As flash is end of life and end of support in 2021 many of you are not able to use the vSphere web client flash-based.
I still have some of my vCenter servers on 6.5 which support the VMware HTML5 web client however to manage some of my NSX 6.3.x task is not supported on it.
I have manged to run vSphere client on my chrome and internet explorer as workaround and below are the steps you need to perform.
https://kb.vmware.com/s/article/78589