The subject of metaphysics is not the result of an abstraction. One cannot abstract the act of being. It is not given, like a nature or a mathematical entity. It is defined in opposition to what is sensible and imaginable. One cannot make a clearing in which being will reveal itself. Anything that appeared in such a clearing would be a nature or a mathematical entity. There is no getting a look at the subject of metaphysics.
So far as the “object of our knowledge” is said without qualification, there is no metaphysical object, but the negation of an object resulting in the thing that metaphysics is concerned with.
But isn’t substance a metaphysical subject, and isn’t it sensible at least per accidens? Yes, but substance is constituted as properly metaphysical only when we judge that it is not limited to the sensible realm. We must explicitly negate the plenitude and sufficiency of sensible and imaginable reality.
But if metaphysics has no objects, how is this different from there being no metaphysical objects at all? Because the very condition of objectivity demands a reality exceeding the object. Our judgment of substance, for example, is possible only on an analogy form our own substantial existence, but it is not given in sensation. Sensation is in fact inadequate to account for an object freely judged to be an object.