Item summon help

hey there, I have a black prism summon but it said I haven't unlocked it yet. may I have some help why I can't summon it now?

you have to have made progress in the aether house before you can use the black prism (specifically, having the egg dexes of nihilego, buzzwole, pheromosa and xurkitree) here's the wiki page on summon items for more info