If you have some virtual desktops in a column, you should be able to switch between them using your mouse wheel. The related settings are placed in Configure Desktop and Wallpaper ... -> Mouse Actions -> Vertical Scroll -> Switch Desktop.
But this feature is not working when you set desktop Layout to Folder View in Wallpaper section of the same window. So if you set Layout to Desktop, it will work as expected, but not the other way.
STEPS TO REPRODUCE
1. Create some virtual desktops beneath each other, like a column.
2. Go to Configure Desktop and Wallpaper ... -> Mouse Actions and make sure Vertical Scroll is set to Switch Desktop.
3. Again right click on desktop and go to Configure Desktop and Wallpaper ... -> Wallpaper and make sure Layout is set to Folder View.
4. Now try to scroll on an empty place of desktop and see there will be no response.
If you scroll on an empty section on desktop, you can't see any feedback.
Virtual desktop must be moved up or down, as you scroll on an empty section of desktop.
Operating System: Arch Linux
KDE Plasma Version: 5.20.0
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.1
Kernel Version: 5.8.14-arch1-1
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-4700HQ CPU @ 2.40GHz
Memory: 7.7 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4600
*** This bug has been marked as a duplicate of bug 419867 ***
*** This bug has been marked as a duplicate of bug 419878 ***