Kant's attitude toward Newton's absolute space is somewhat confused. At times he defends the absoluteness... At other times he presents his own arguments in favor of the relativity of space and motion. ...At any rate the problem of the absoluteness of space and time in classical science refers not to the essence of space and time (a problem which would degenerate into one of metaphysics, hence would be meaningless to the scientists), but solely to a discussion of those conceptions which are demanded of the world of experience. Hence we may realise that a man ignorant of mechanics is in no position to pass an opinion one way or the other. And Kant's knowledge of Newtonian mechanics was extremely poor, to say the least.
A. D'Abro, The Evolution of Scientific Thought from Newton to Einstein (1927) footnote, p. 417-418