I'm playing Oblivion. When trying to unequip an item, it tells me I can't. Why?

Oblivion Elderscrolls is what I'm playing. I'm trying to unequip the item Steel Claymore and equip the item Blade of Woe. I haven't cheated, and I'm not fighting,riding, running, walking, or any of that when I try to change weapons. Why isn't this working?