(My reading relies heavily on Pinkard's Sociality of Reason. Yes, I will be happy if I can comprehend a single chapter of a secondary source on Hegel.)
So there are things and properties of things. The properties are what sense-certainty tries to get a hold of in complete independence from any higher order apprehension (that is, in abstraction from perception and everything which contextualizes perception itself). Now, they are perceived not as detached units but as properties of a thing perceived through them. But how does perception get a hold of this thing in which the properties inhere? That is, how can this thing be the object of perception? It is not itself a property, so it does not have the relatedness to a thing which mediates universal properties. Then it is a self-related individual, a one. As such it does not have the properties which would differentiate it from other ones. Thus it is a thing with properties and it is a thing without properties.
The attempt to reconcile this contradiction by way of a distinction between primary and secondary qualities fails because it implies that the thing is not the universal medium of the properties, but rather perception itself is responsible for their unity. The thing itself with its actual properties (primary qualities) on this account must lie beyond the reach of perception. This conclusion is not only epistemologically frustrating but incoherent: there remains nothing for perception to attribute that second set of properties to, even mistakenly. No thing in perception means no thing for perception to be in error about.
The properties which we perceive must be perceived as the properties of the one thing. And only by way of these properties can we perceive the thing as a determinate one differentiated from other ones. These propositions are the product of the experience of sense-certainty, which demonstrated the impossibility of unmediated knowledge of particulars. The perception of the thing as a one which as such has no properties essential to it falls back into the same impossibility, since it would have to know its object in exactly the same way, as a blank, undifferentiated singular thing.