Board Thread:Game Discussions/@comment-73.81.159.86-20170227155505/@comment-4199297-20170227171556

They changed the fuse system (you can now only fuse together of the same rarity) and apparently didn't bother to check if that impacted any quests. The only thing you can do is send in a ticket at https://games.gree-support.net/hc/en-us and hope they change the quest in a future update. Up to that point, you won't be able to complete that quest.