Kant argues that the aim of the state should not be happiness but freedom. What does he mean by this? Is this fundamentally different from or the same as Hobbes’s view that the sovereign should guarantee individual security?

For Kant, the highest good is freedom because it is only when people are free that they can make unconstrained moral choices. Something may only be considered a moral act if it is willed as a moral act. For Kant, therefore, a firefighter saving someone as part of paid employment is not performing a moral act, while a neighbor running into a burning building to save a child is performing a moral act. It is intention that determines the moral quality of an act, not the consequence. The state has two roles within this system. First, it should not constrain people in ways that limit their freedom to act in a moral fashion and make moral decisions. The state must, however, hinder people from acting in ways that constrain the moral freedom of others. For Kant, happiness is too vague and variable a concept to be the goal of the state.
This is quite different from Hobbes, who sees humans as rationally self-interested and who sees the role of the state as not preserving freedom. Rather, the state is the result of a contract in which individuals sacrifice freedom for security.
https://plato.stanford.edu/entries/kant-social-political/