I think what Locke is saying is that some people operate with a notion (which they call 'substance') of a kind of substrate in which properties inhere. But they can't say anything more about this supposed 'substrate'. So it's too vague a notion to be really meaningful or useful.
Comments?
Is a thing simply a set of qualities?
If so, can we conceive of a Universe in which qualities didn't go about in packages?
If so, is there a question: in this Universe, what brings it about that they do go about in packages? Is something tying the packages together?
VP