There is currently no Synology Plex Server connected to your user account.
A Synology server was today deleted by you, most likely via this page: https://app.plex.tv/desktop/#!/settings/devices/pms
Which means that you need to re-claim this server into your user account, in order for your various Plex clients to find/use it.
https://support.plex.tv/articles/204281528-why-am-i-locked-out-of-server-settings-and-how-do-i-get-in/
