You can only trade in 6 cores per Advent event. Your loss is, ultimately, inconsequential. You missed out on a gold token and a small chance for a SR/SSR weapaon or eidolon. Heartbreaking, I'm sure.
well, the event isn't over yet, so as long as it'll be my only lose it's alright i guess.
not that i need any of the SSR weapons/eidolon, but sr material to level up skill is nice, still low chance though so i most likely wouldn't find it anyway.