Hello, please read here: Synology installation changes for Plex Media Server
In a nutshell:
To install the 64-bit version of Plex Media Server (don’t worry, your server/library data won’t be affected by following these instructions):
Stop PMS
Uninstall the unknown or 32-bit PMS
Ensure that you are running DSM 6.0 or newer
Download the 64-bit version of PMS (https://www.plex.tv/media-server-downloads/219)
Install the server
Restart / Reboot DSM
