One of the best-known advocates of UBI/negative income tax was Milton Friedman, and he most definitely was a libertarian.

There's a difference between idealistic versions of political philosophies that don't match human behavior, and practical versions of those philosophies that do.

Let's say we wind up with 50 million people who have no marketable skill. What's your solution? Let them starve? What are you going to do when they take exception to that?

If your answer is to simply kill them, good enough, but you need to face up to the fact that this is what you're advocating.

Jailing them is not an option -- that will likely cost more than the UBI scheme, and you can't even put them to work (it being a given that they can't perform any job that provides a net economic gain).