I've tried to install plugin into Plex from github, but I failed.
I tried ubuntu cli commands which I reviewed from plex FAQ but it doesn't work
Could someone provide me with info how to install plugin from github
simple answer: you can't
apps are preconfigured packages. if it's not included by the creator of the app you can't add it manually.
You could fork the app, include the packages you want and install your own image. But if you have no experience in building apps it's unlikely you'll succeed with that.