Is there a word for the 'art' of creating amulets?
I'm not an expert on fields of magic but in a couple of cases where such a word would likely be used, even if uncommon, the authors chose "crafting". It was obvious from context and some uses that they meant crafting magical items. Even so this is evidence of the lack of such a word as much as it's a suggestion.
Try making Phylactery into what you need, like Phylacterism
(nb: Although D&D has a very narrow usage of Phylactery, it can mean any amulet or charm).