1. use priority not Priority
2. start with priority=0
Code:
sets.precast.FC = {
right_ring={ name="Meridian Ring", priority=12},
left_ring={ name="K'ayres Ring", priority=11},
body={ name="Reverence surcoat +2", priority=10},
hands={ name="Souv. Handsch. +1", augments={'HP+105','Enmity+9','Potency of "Cure" effect received +15%'}, priority=9},
ammo={ name="Egoist's Tathlum", priority=8},
right_ear={ name="Odnowa Earring +1", priority=7},
left_ear={ name="Thureous Earring", priority=6},
waist={ name="Creed Baudrier", priority=5},
head={ name="Carmine Mask", priority=4},
back={ name="Rudianos's Mantle", augments={'HP+60','HP+20','"Fast Cast"+10'} ,priority=3},
legs={ name="Odyssean Cuisses", augments={'Weapon skill damage +1%','"Mag.Atk.Bns."+3','"Fast Cast"+7','Mag. Acc.+7 "Mag.Atk.Bns."+7'} ,priority=2},
feet={ name="Odyssean Greaves", augments={'Mag. Acc.+21','"Fast Cast"+4','INT+7','"Mag.Atk.Bns."+5'} ,priority=1},
neck={ name="Baetyl Pendant" ,priority=0},}