So the robe is repairable (but not on TC1) but you still cannot apply powder of fortification to any items that normally did not have durability or jewelry.
Items with durability that do not have any tags to prevent powder of fortification are still not powder-able: Talismans, footwear items, Robes/epaulettes, Paladin Swords, clean jewelry including artifacts like Ring of the Soul-binder, & Enchanted Coral Bracelet.
If the item(s) have durability and has no tags (Brittle, Replica, Imbued, & Cannot be Repaired) that prevent powder of fortification from being applied, it needs to be powder-able.