e.g. If I could know what decisions you were making (a notion that is at least theoretically possible, if the universe were truly deterministic), I could analyze it and predict the answer you would give to a particular question, even if I told you truthfully what the answer to that question were going to be.... however, with your so-called illusion of free will, you could utilize the information that I gave you in the present about your alleged future action, and then deliberately contradict it, invalidating the prediction that I made, meaning that my knowledge about the future state was incorrect, which leads one inescapably to the conclusion that even if the universe is deterministic, it is impossible to know it.
If you know the state and rules of the deterministic universe and use that knowledge to predict my decision, you must also in making that prediction predict your own actions, such as telling me your predictions, and my reaction to your actions. So I cannot contradict your actual prediction, unless you weren't really using the state and rules of the deterministic universe to make your prediction. (Of course I could contradict your announced prediction, if it were other than your actual prediction.)
It seems counter-intuitive that I would be unable to make a decision other than the decision you tell me I'm going to make, but if you grant that the universe is deterministic and has consistent rules, and your are able to know enough of the state and rules to make accurate local predictions, then that must be the case.
Arguing that such a universe cannot exist (or cannot be known) because it violates your intuition about free will is begging the question.