Can't Parent one bone to another - this option is inactive I'm trying to create a pop-up book, using this lesson: https://www.youtube.com/watch?v=I6GjavMTaiQ 

But I can't Parent bone that moves the page of the book to the bone that moves the cover of the book - in the setting of the bone, this option is inactive:



I tried the usual parent (Ctrl+P) but in this case, the position of the bone that moves the page, is not recorded in the library of poses, when I move the bone that moves the cover of the book.
If you want a bone to have a parent bone you have to do the parenting while in 'Edit Mode'. At the minute you are in 'Object Mode'. From the header of the 3D view, with the armature selected, choose 'Edit Mode':



(The bone you want to be the parent will also have to be in the same armature for it to be available as the parent).