The Categorical Imperative and Deduction

Kant makes clear that no deduction of the moral law is either possible nor necessary. He makes this claim on account of a distinction between our theoretical and practical faculties of reason. However, Kant has explained that there is another process for arriving at a conclusion regarding the objective reality of the moral law by use of a "warrant" or "credential" that allows us to make this move. This has further implications for Kant's argument, as he will use it as an attempt to break the notorious circle of the Groundwork of the Metaphysics of Morals. Why such deduction cannot (nor needs to) needs to be explained further before we may move towards the other points of his argument in proving the objective reality of the moral law.