Gallery of Greats

Hi there. Does that happen with either one of those? Also, is the item in another set or lineup?

One of the two Bucyk cards is BND and the other one is in your active lineup. That's why you are unable to add those cards to the set.

If you watch the video carefully enough, you can see that one of the two Bucyk cards is BND and the other one is in the active lineup, so there is the answer to OP's question.

And by the way, I'm pretty sure an item can't be in a set an in the collection at the same time.

that BND (LAC on the French version) Indicator on the cards means that you cannot trade them or use them to complete a collection.