In DSM 7, Plex Media Server runs under a new user, PlexMediaServer, instead of Plex. Also, PlexMediaServer is a System Internal User, a new user type Synology added in DSM 7.
Also, the Plex Data Folder is now /PlexMediaServer instead of /Plex.
Since you’ve already installed PMS on the system after updating to DSM 7, the PlexMediaServer user already exists.
If the /PlexMediaServer shared folder still exists, delete any contents. It needs to be empty.
Give user PlexMediaServer at least read only access to your media files. See FAQ29.
Install Plex Media Server on the NAS. It is OK to use the version in Synology Package Center.
You can upgrade later if desired.
When installing, choose the Plex Claim Token option. It successful, you will see a message saying “Plex Media Server successfully claimed by username” (or something very similar).
Login to the Plex server using the IP address of the NAS. For example, if the IP address is 10.1.2.3, use http://10.1.2.3:32400/web.
You should be able to access server settings, create libraries, etc.
After initial access via IP address, you can access via https://app.plex.tv/desktop, etc.
