I have the app Plex Media Server in my Synology DS220+ installed, however, when I first configured it I used a gmail institutional account which was cancelled. The problem is that I forgot the password of that account, and therefore, I cannot link my NAS to another gmail account.
Uninstall Plex Media Server, using the Uninstall Only (first) option.
Re-install the same version, using the Plex Claim Token option.
The installation will lead you through obtaining and using a claim token to claim the Plex server.
Option 3: Log Out / Log In
You must be able to login using the old Plex account.
Login to the server using the old account.
Navigate to Settings → Server_Name → General
You’ll see “Server Signed In As old_e-mail_address”
Sign Out of the server.
Sign out of the old Plex account
Upper right corner, click on user symbol, choose Sign Out from drop down menu.
Quit the browser (close all windows).
The server is now in an unclaimed state. To claim.
Launch a browser and point it at the IP address of the NAS: http://syno_ip_address:32400/web.
For example, if the NAS address is 10.1.2.3, use http://10.1.2.3:32400/web.