Please follow FAQ 31: Reset / Reclaim PMS on DSM 7 (Not Authorized - Locked Out).
If it does not complete successfully, write back with any error messages, etc.
If it does complete successfully, you should be able to access Plex Media Server.
Do not use the Open button in Package Center (at least initially).
Point a private/incognito browser window at the IP address of the NAS. For example, if the IP address is 10.1.2.3, use http://10.1.2.3:32400/web. You should be able to login and access the server. You may see a “not hosted at Plex” message. That is OK. It just means you are connecting directly to your own server.
FYI, In addition to the FAQ, here’s another document to help with running Plex on Synology NAS: Installation & Setup of Plex Media Server on Synology
