KDE plasma panel does not show menu: move to other desktop workspace
when right-clicking on the icon of running application on panel bar, nor it shows pager (switching desktop) on it

How to fix this ?
Note : This is under condition the OS is installed with 2 DE in same time & user account,
LXQt, though it has desktop workspace icon shown on panel (ie. not broken)

do you mean the Desktop Pager? That is a widget you need to install in the Panel. Not all distributions come with more than the default virtual desktop, btw.

FWIW: please always state your exact Plasma version and distribution and whether/what changes you made to the default setup.

FWIW2: you can select the distribution in your user profile....