Due to how DSM 7 works, clicking the Open button in Package Center, using app.plex.tv, etc will not work when trying to claim a server.
Also, note that PMS 1.21.0-3744 is outdated. It was a beta release for DSM compatibility. Unfortunately, Synology will not post a current version via Package Center.
Use Intel 64-bit. The AMD Ryzen V1500B is the same architecture.
Note the separate download for DSM 7. Installing the DSM 6 version on a DSM 7 system doesn’t hurt anything, but the install will immediately fail.
When you update to a current version, you’ll see a new shared folder, /PlexMediaServer. This is the Plex Data Folder location, the equivalent of /Plex on DSM 6 systems.