Some additional info…
From a system on the same subnet as the NAS, point a browser at the IP address of the NAS.
For example, if the IP address is 10.1.2.3, use https://10.1.2.3:32400/web.
See if you can login and claim the NAS.
If that does not work, then start with checking the basics.
If the NAS cannot communicate with hosts at plex.tv, then it cannot be claimed.
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Verify the network settings are correct: IP address & mask and default gateway.
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Configure the NAS to use a public DNS server: 1.1.1.1 (Cloudflare), 8.8.8.8/8.8.4.4 (Google), or 9.9.9.9 (Quad9). ISP DNS servers sometimes have trouble resolving plex.tv addresses.
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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+, you can claim the server by uninstalling/re-installing Plex Media Server. See Synology FAQ 31: Reset / Reclaim PMS on DSM 7 (Not Authorized - Locked Out).
