Full Idea
Kant no longer had the right to his distinction between 'appearance' and 'thing in itself' - …insofar as he rejected as impermissible the inference to a cause of the appearance, in accord with his account of causality.
Gist of Idea
Kant can't distinguish appearance from thing-in-itself, because he rejected the inference needed
Source
comment on Immanuel Kant (Critique of Pure Reason [1781]) by Friedrich Nietzsche - Fragments from 1886-87 (v 17) 5[004]
A Reaction
This is the core of the debate over whether Kant is an idealist. Nietzsche thinks he is, and hence entirely rejects his basic ontology.
Book Reference
Nietzsche,Friedrich: 'Fragments from 1886-87 (v 17)', ed/tr. Leiner, George H. [Stanford 2025], p.3