I completed the event and have a yellow card (Lily the courtesan) which i am trying to upgrade - i have 11,000/4,500 and everytime i try and fuse, it says get stones

How do i fuse this card? I have more than enough

I'm not 100% sure, but I think you have to get a mythic card from the event. It will automatically upgrade then (at least that's what has happened to me.