You are not adding any child to the box. You need to create a widget (a buff widget) and plug the result there.
If your buffs are categorized by rarity, make one array of buffs for each rarity.
Shuffle and Index are here to prevent picking the same element twice.
If, for each box, you reshuffle and pick the first element, then you risk having two boxes with the same item. Alternatively you can remove item from the array after picking it, but there’s no much reason to do that over shuffling just once.
Here’s a less confusing version maybe, that doesn’t use for loop. It ends up duplicating lots of nodes, but maybe easier to understand for you :
