How to add bone to Skeleton3D?

I Literally CANNOT FIND HOW TO ADD BONE!!! Im adding BoneAttachment3D, but it doesnt wanna to edit in size, it says “Node BoneAttachment3D not linked to any bone!” THEN HOW IM GONNA LINK IT TO BONE AND HOW I CAN GIVE A NAME TO BONE???

BoneAttachment works on existing bones it does not create new ones. I am confident Godot does not support editing Skeletons in-editor, you will have to use a 3D modeling program like Blender to do so.