This usually indicates a network issue.
Plex Media Server must be able to communicate with plex.tv to claim a server. If it cannot, claiming will fail (unfortunately, Plex does not provide any feedback other than doing nothing).
See this post and the one below it. The thread references DSM 7.2.2, but the info applies to any version of DSM: Issue Running Plex on Synology DSM 7.2.2-72803 - #6 by ChuckPa
