Since no one bothers to scroll up or check 1-2 pages back, I copied an old response this once:
"If you can use your armor for fusing/enhancing, one of your knights must be wearing it. If it's not on one of them at either Arena, Epic Boss or Adventure, then one of your knights must be wearing it for summoning stone fights. They're working on fixing the accessibility of the summoning stone, so if this is the case either wait for the next update to happen or spend 4 gems to take it off."
Guess this can be used as a standard copy/paste respond every so many pages.
I'm pretty sure there's not (yet).