The third sentence is puzzling. I can speak signs but not express them? What's the difference? Perhaps what this means is: I cannot mean things, since meaning in the relevant sense is not personal or psychological. “The cat sat on the mat” means what it means, regardless of me. It is signs that mean things, not people that do so. We can only use or utter signs. I can say “By ‘cat’ I mean dog” and then say “The cat sat on the mat” meaning “The dog sat on the mat,” but ‘my’ meaning this is done by the words themselves. I cannot mean something without words, i.e. without signs or tokens of some kind, be they (mental or physical) pictures or words or whatever. But this seems like a stretch.
What about the last sentence of 3.221? A sentence consists of signs, not of objects. It can say, in effect, “The thing designated by ‘cat’ sat on the mat.” It cannot incorporate a non-linguistic object into itself and say this is what ‘cat’ or ‘Fluffy’ stands for. Why not? Because to do so is to treat or make the object in question part of language, hence linguistic in the relevant sense. Holding up a cat and saying “This is Fluffy,” meaning “The meaning of ‘Fluffy’ is this,” is giving a definition, performing a linguistic act (an act, that is, within or belonging to language). It is, in effect, a kind of equation: Fluffy = ö. And equations tell us nothing, including what a thing is. I can of course say that Fluffy is a cat, is seriously overweight, is cowardly, and so on. But this tells you how Fluffy is, not what Fluffy is like.
What else is there to say though? What would it be to say what something is in some other sense? Nothing at all. That is why it cannot be done. The “impossibility” is logical, i.e. there is no such thing as doing that, i.e. the very idea is plain nonsense. This reminds me of Berkeley. We can say that the apple is red, mealy, soft, and so on, but we are not adding anything to say then that it is matter. Or, for that matter, idea.