The base-Node of a Bone-Structure always need to be an Skeleton2D.
As childs you can use multiple Sprite2D and Bone2D to build up your animated asset.
A little bit hidden, but at the top of your Godot-Editor, is a toolbar for easy creating Bone2D from Node2D .. and Skeleton2D also have the Skeleton-Option " Reset to Rest Pose ".
After properly configuring the Rest-Position in each Bone2D, and resetting the skeleton, you are ready to go.
Hope i could help someone in future,