You are running into two issues: First, updating from DSM 6 to DSM 7 forced Plex to make changes in how PMS is installed; Second, the buggy DSM 7.1.1 Update 7 and corresponding Package Center problems.
DSM & Package Center
If your NAS supports DSM 7.2 or DSM 7.2.2, then updating to one of those versions of DSM resolves the problems with Package Center. If you update to DSM 7.2.2, then you must download and install the DSM 7.2.2 specific version of Plex Media Server from plex.tv.
If staying on DSM 7.1.1 Update 7, use the Linux command line (SSH) to start/stop/uninstall Plex Media Server. See the thread linked by @tom80H for details.
Restoring old (DSM6) Plex Media Server
If you still have the /Plex shared folder from when the NAS was running DSM 6.x, then it is possible to recover your old Plex Media Server. See Struggling with migration from DSM 6 to DSM7 - #2 by FordGuy61.
If running DSM 7.1.1 Update 7, you will have to use the command line to stop/uninstall Plex Media Server.
In Step 5, you will not be able to uninstall & erase. Instead, uninstall using the command line, then, using File Manager, delete the contents of the PlexMediaServer (not Plex) shared folder.
Completely Removing Plex Media Server
If you wish to completely remove Plex Media Server and are running DSM 7.1.1 Update 7:
- Stop Plex Media Server via the command line (synopkg stop …).
- Uninstall Plex Media Server via the command line (synopkg uninstall …)
- Control Panel → Shared Folders: Delete the PlexMediaServer shared folder. If the Plex shared folder exists (from DSM 6), then delete it as well.
Note 1: Use sudo to run synopkg as root. You will need the admin password for the NAS.
Note 2: synopkg has many options. Issue the command without any arguments to see all the options ($ synopkg).
