I'm not able to add media library in my account

Plex Media Server Metadata & Adding Files
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I’m not able to add media library in my account

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In order to complete the setup you need to open your Plex Media Server through its local IP.

If you’re on the same device… open http://127.0.0.1:32400/web in your browser. If you’re on a different machine inside your home network, use http://[PMS IP address]:32400/web instead. This will bring up a wizard/dialog where you can name your server and link it to your Plex account (as well as creating your first libraries).

Once you “claimed” your server (=linked it to your Plex account), you’ll also be able to access it from other Plex apps or via http://app.plex.tv/desktop.

https://support.plex.tv/articles/200288586-installation/

If you haven’t already claimed your server, you should also be able to manage it using the local interface (incl. adding new libraries)