Guild Wars 2

Hi,
I’ve tried to transmute the halloween book skin onto my ascended back item, but the Confirm-Button is always greyed out. As you can see in the screenshot, I am using a “fine transmutation stone” for level 80 equip, so that’s not the problem.

Having the same problem with the Tome of the Rubicon.
I thought it was because I upgraded it with an infusion but I dunno now.