insufficient attunement slots
Dark Souls 3: What do the stats and attributes do? EXPLAINED! ...
Attunement Slots Fire attack The level of fire damage inflicted. Casting Speed Speed at which spells can be cast. Lightning attack The level of lightning damage inflicted.
D&D 5E - Thoughts Regarding the Number of Attuned Items | EN World ...
If you're a spellblade type, and you want to go Sword-Focus, that leaves you with two of your attunement slots consumed just with your hands.
Attunement Slots - Darksouls3 - darksouls3.wikidot.com
Selecting a new starting Attunement, you haven't unlocked yet on another slot will permanently unlock the Attunement across all your slots.