Are you using an outdated version? Starting with the April 2nd update, you now sit automatically and sneak to stand back up.

/trigger sit



then look down at the invisible chair (llama) and right click

Google Translate says, you can't sit still?

The chair is actually a LLama (before typing this, I actually fixed it so it wouldn't be visible). It's not walking away is it?

You're player may rarely get pushed away by it, before sitting. You just have to right click where you summoned it.