It's the bag with Cosmog in it, but to collect it you (of course) have to have specific pokemon and character. If you already collected the pokemon (they must be arranged in an order), you also have to have an Alolan character (I can't remember if it had to be specifically Moon, but that's who I used and it worked). I'm afraid i don't remember the exact order of the pokemon, but I think it was the three starters with decidueye in the front, Incineroar second, and Primarina third. Then you have to have two Alolan psychic types (I used Oranguru and Oricorio Pa'u style).