Advice Request - Glamor Plates

[Carin-Eri]

Looking for advice if possible please?

I've been levelling my character through the Red Mage job and hit level 60 earlier. I purchased the Serpentskin Tights of Casting to use and placed said item in the Glamour dresser. However, when I tried applying it, and tried to save that plate, it wouldn't accept the Tights as part of the outfit I was trying to save.
So far as I can tell, Red Mage should be able use it, evidenced by the fact that I can equip it outside the dresser, but I don't seem able to save it as part of a new plate.

Could anyone please suggest what I am doing wrong? I have to confess to being pretty new to the whole 'Glamour plate' thing and am wondering whether I've overlooked something, particularly since (as noted) I can equip the item outside the Glam dresser.