Why can't I add BONES to image on Anime Pro Studio 7?

I've seen how to videos and I know what to do, but like me and so many other people they have the skeleton set up to attach to the image but when they follow the instruction the studio blocks or doesn't allow you to add the skeleton to the image. Does anyone have a answer why? I would really like to start animating but this problem is stopping me. I had this problem adding vectors before but somehow I fixed it.