The total looks too small
Some folders are private to macOS until an app has Full Disk Access. Grant it in System Settings → Privacy & Security → Full Disk Access, then scan again. DiskDuo shows a banner while it is missing.
The permissions list is empty
The same cause: macOS keeps those lists where only an app with Full Disk Access can read them. DiskDuo says so rather than implying nothing is granted.
A scan is slow
A cold cache, a nearly full disk, another program writing hard to the same disk, or a network volume will all slow it. Exclude what you do not need in Settings, and scan a folder rather than the whole Mac when you already know where to look.
The menu bar popover will not open
Turn the menu bar helper off and on again in Settings. The popover sits at the level menus draw at and appears above other apps, including full-screen ones; if it does not, please write.
Something else
Write to me at me@shaheermalik.com with what you did and what happened. One person reads that, and it is the same person who can fix it.