I'm not too sure what else it could be, unless you haven't initiated a session with loot locker.
if you're basing your code off of the LootLocker website, this video might help https://www.bing.com/videos/search?q=lootlocker+leaderboard+tutorial&&view=detai..., as it goes far more indepth. But beyond that, i'm afraid i don't know how else to help