I've found that virtual folder creation is sometimes impossible unless I quit and restart theBrain. I'm just adding this in case other users are having similar problems. This is a great feature and it really is simple as just dragging in a folder, assuming preferences are set correctly. If it doesn't work, try quitting and then restarting. 
OSX 10.13 
iMac (Retina 5K, 27-inch, Late 2015)
3.3 GHz Intel Core i5
32 GB 1867 MHz DDR3

I haven't found any issue with this. Does this happen with the Virtual Folder option checked?

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