Assuming you’re not setting the server up for public access…
This is a new installation, correct? No play history, etc to save. So, just completely remove Plex and start over from scratch.
Per FAQ #9:
- Package Center - Remove the Plex package
- Control Panel → Users - Delete user Plex
- Control Panel → Shared Folders - Delete the Plex share
- Restart DSM
Then…
- Make sure the Synology and PC you’ll use for setup are on the same RFC 1918 IPv4 compliant network.
- Read Installation & Setup of Plex Media Server on Synology
- Download the latest public Plex Media Server release from plex.tv/download (the version available from Synology is a year old).
- Perform a manual installation per the Installation & Setup document.