Wolrdbuilding discussions here.

Imagine you basicaly have a multiplanar organisation focused on, optimistic and sciency and whatever. They meet other civilisations, invite their most brilliant researchers to help and join. The semi-meritocratic and semi-oligarchic council seeks new members from across all muniverse to further unite the planes within the Federation, albeit subjected to some tight standards.

Point if, how is a civilisation of libertarian "muh" freedom is going to survive without a proper legal system. The only crime considered is abuse of free will (slavert, vivissection, et cetera), with the overall focus being more on preventing causes of crimes rather punishing the murder or rapist. It advocates for a minimal inteference ffrom the government, whch makes it particularly hard t control crimes of passion like murder.

How does this work as a conceivable civilisation?

Just going after the causes of crimes inches this away from libertarianism. Libertarians recognize few crimes and are reluctant to enforce systematic changes. ... Actually, what you mean by "preventing the causes of crimes" is confusing me. Would that mean you don't take the murderer off the streets, or you would, but not for the sake of punishment?
How would it work without a proper legal system? I think the system would be subverted pretty rapidly, unless the society was really, really good at removing the causes of crimes. Short term it could work. Long term, I think it would collapse in a big way.

Just saying, the federation is a socialist state of limitless resources.
Nobody wants for anything because replicators meet literally any need and the government provides everything for their group.

There's no such thing as an economy because nothing has any value.
Aside from crimes of passion, that eliminates all crime motivated by financial issues and possession.
Crimes of passion are largely avoided by indoctrination and brainwashing.

Left-libertarianism isn't necessarily opposed to government. They just want to keep the government as minimal as possible, because they believe that government favors the powerful. Courts are something that you could keep. They would have minimal power, since pretty much all cases they handle will be between two people, not between a person and the state.

Preventing crimes of passion is impossible.

I forgot to clarify.

This is a general deconstruction of the modern Federation archetype, not the Star Trek one. Many modern sci-fi Federerations tend to focus on the "passion and creativity" side than the socialist one.