Is your server claimed?
Point a browser at the IP address of the NAS: https://syno_ip_address:32400/identity (ex: https://10.1.2.3:32400/identity). You’ll see a line of XML with claimed="0" or claimed="1". 0 = unclaimed, 1 = claimed.
If it is not claimed, then you will see the Update Available message.
If it is claimed, check the Server Update Channel setting (Settings → Server_Name → General). Ensure it is set to Public, not Beta, as you are running the current public release.
Regarding server claiming…
Many server claiming issues are networking problems. The server must be able to communicate with hosts at plex.tv for the process to work. If unable to claim, check the network basics such as IP address & mask and default gateway. Using a public, vs ISP, DNS server can also help sometimes. Try 1.1.1.1 (Cloudflare), 8.8.8.8/8.8.4.4 (Google), or 9.9.9.9 (Quad9).
Also, verify the time is correctly set on the NAS. If the time is incorrect, claiming may not work. Sync the NAS with a public NTP server to ensure time is correctly set: DSM Control Panel → Regional Options → Synchronize with NTP server.
If running DSM 7, claiming the server is easily accomplished by uninstalling/re-installing Plex Media Server (i.e. no need to use SSH & claiming script). See Synology FAQ 31: Reset / Reclaim PMS on DSM 7 (Not Authorized - Locked Out).
Regarding DSM 7.2.2+…
Updating from previous DSM 7 version: DSM 7.2.2 forced a packaging change for Plex. You’ll need to install the DSM 7.2.2 specific package after updating DSM. See Synology FAQ 34: Upgrading Plex to DSM 7.2.2+.
Updating from DSM 6: Several steps required. Follow the FAQ to avoid headaches. If the process goes sideways, don’t panic and wipe everything. It is usually possible to recover from a failed migration. Synology FAQ 27 - Migrating an existing PMS from DSM 6 to DSM 7.
