The section of code you have under "Code to add subfolder" needs to use the same objects that you used in the section labeled "QCResourceFolderFactory Object". The TreeManager object is for folders in the Test Plan.
If newFolder is pointing to the folder to which you want to add a subfolder, then you can do this
set subFolder = newFolder.QCResourceFolderFactor.AddItem(null)
subfolder.ParentID = newFolder.ID
subfolder.Name="AbhishekSubFolder"
subfolder.Post()
If newFolder is pointing to the folder to which you want to add a subfolder, then you can do this
set subFolder = newFolder.QCResourceFolderFactor.AddItem(null)
subfolder.ParentID = newFolder.ID
subfolder.Name="AbhishekSubFolder"
subfolder.Post()