Your post is very scant on information (you may be interested in How to Write a Good Bug Report), but I’m going to go out on a limb and suggest that you:
- are trying to extract the power from a legendary item that doesn’t have one; or
- have extracted the legendary power from an item that your current class can’t equip (the Demon Hunter can’t equip The Furnace, for example, nor can they use it in the Cube); or
- have an item in the cube that you also have equipped.
If you could provide more details in your post, that would be incredibly helpful 