General web search for non-Duplicati USN change journal use (such as fsutil usn readJournal) says it needs Administrator privileges, and if Duplicati is subject to the same controls, this means either install as a Windows service, or run Duplicati as a user in the Administrators group. If you run with the automatic start-at-logon tray icon option, the shortcut lives in C:\ProgramData\Microsoft\Windows\Start Menu\Programs\StartUp and you would Right-click → Shortcut → Advanced → Run as Administrator. Because it’s a shared setting, any users who are not Administrators might get asked at login to enter a password for an Administrator user.
Not ideal, but perhaps you or someone else can figure out if this is necessary, and if there’s a better setup…
If you take the Windows service route, which requires manual work, life for this (and snapshot-policy) is easy: