Duplicati must run with elevated rights for it to be able to utilize snapshots. If you have a default Duplicati install that runs under your user process, it won’t have permission.
One method is to switch to running Duplicati as a service. This may require you to move your Duplicati configuration and sqlite folder.
An alternative (the one I use) is to leave Duplicati as something that runs in my profile, but instead of using the shortcut in Startup, use Task Manager to launch it. You can configure a task to be triggered at your user logon, and the task can be configured to run with elevated rights (without a UAC prompt).