“Immanuel Kant was an advocate of liberalism and nationalism.”

The statement is half true, and the false half matters.

Kant as a liberal — established. What is Enlightenment? (1784) defines enlightenment as release from self-incurred immaturity, with the public use of reason as its condition. The categorical imperative requires treating humanity always as an end, grounding rights in reason rather than custom. In Perpetual Peace (1795) he sets out freedom, equality and independence as the principles of the civil constitution and argues for a republican order with separation of powers.

Kant as a nationalist — false. He is a cosmopolitan and an explicit critic of the national state as a final unit. Perpetual Peace proposes a federation of free states precisely to end war between nations; he advances a right of world citizenship limited to universal hospitality, and condemns European colonial conduct.

Where the confusion comes from. German nationalism drew on Herder's Volk and on Fichte's Addresses to the German Nation (1808). Fichte began as a Kantian and transferred the Kantian vocabulary of autonomy from the individual to the nation. That transfer is a use made of Kant, not his own position.

Conclusion. Kant is a foundational liberal and a foundational cosmopolitan; calling him a nationalist mistakes his influence for his argument.