I see several threads related to this problem. When trying to apply certain weapon enchants, you can run into the error of the title.
It is not directly related to heirlooms or vellums, as the other threads suggest. I was using a low-level set of white gear to level enchanting on my demon hunter. I could not enchant any 2-hander until I unequipped my own gear.
IOW, trying to enchant a weapon in your bags seems to check your wielded weapons first for compatibility with the spell, or something to that effect.