If any one knows how do I install plugin. I am unable to install
Only plugins that you have acquired via the Unreal marketplace can be installed by the launcher or better, via the vault. There you can add the plugin either to your engine, to make it available for all projects or the engine itself, or just to a single project that needs this specific plugin.
A plugin that came from other sources like Github needs to be copied to the following folders, based if it usage:
Example for Project plugins (created folder Plugins if needed)
x:\ProjectFolder\Plugins\YourPluginFolder
Example for engine plugins, or plugins for all your projects ((created folder Marketplace if needed)
C:\Program Files\Epic Games\UE_5.4\Engine\Plugins\Marketplace\YourPluginFolder
Plugins that you add to your engine have to be in the folder Marketplace to get loaded by the engine.