It is not possible to Add Permissions on folders under Global Collection Hierarchy

Document ID : KB000111451
Last Modified Date : 16/08/2018
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If you intend to organize your global collections using folders, set up the OneClick Navigation panel with a Global Collection Hierarchy.
In this Global Collection Hierarchy, you can create multiple levels of folders to represent previously defined global collections.
However permissions still need to be granted at Global Collection level.
Is it possible to add security string to Global Collection Hierarchy folders?
Spectrum 10.x
This is not possible since the Global Collection Hierarchy Folders are not created as models in the SpectroSERVER database. The Global Collection Folders are stored in the CollectionHierarchy attribute (0x12ad7) on the GlobalConfig model on the SpectroSERVER. 
Therefore, the "Security_String" attribute (0x10009) does not exist on the Folder itself. You can apply the Security String to the Global Collection model, or the model that is a member of the Global Collection, but not to the Global Collection Hierarchy Folder. 
Additional Information:

Create a Global Collection Hierarchy