Well, Kant was not around to know about the left-right brain interaction. That psychological theory didn't come on the scene until hundreds of years after he died.
Nevertheless, in his moral philosophy the "ought" as prescribed by the Categorical Imperative implies a "can". So, for Kant *anything* that practical reason prescribes as an "ought" *can* be done. Outside of that, I'm sure there's lots of room for things we "cannot" do, if you mean physical incapability.