There's a version of libertarianism where if you rightfully (via all-voluntary process) own a piece of land and make a kingdom there, or create a network of agreements leading to monarchy, it's fine.

The issue is that you can never own territory fully logically rightfully by even Rothbard, so you only can have a voluntary monarchy without territory cause territory is inherently commonly owned, or owned by nobody. A group of people needing some territory to live on can claim as much as they need, but one person who wants to have a kingdom on it - nah. And a voluntary monarchy is not much different from a BDSM orgy, you can have it right now if everybody agrees.

So bog standard ancap is incompatible with this, but there are a few kinds of fascists (or "paleo-libertarians") who have a set of ideas which isn't.