Why my hide leaderboard don't work?

Yo Problem

Your main problem is that you're trying to use the StarterGui, when you need to use the Players PlayerGui. Also, you also need to make sure the PlayerGui is there before you do this.

Fix Dis Stauf

So let's try this script:

while game.StarterGui:GetCoreGuiEnabled(Enum.CoreGuiType.PlayerList)==true do
    game.StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.PlayerList, false) --Player list
    game.StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Health, false) --Health
    game.StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Chat, false) --Chat
    wait()
end

(Tested and Works) - Put in a LocalScript

Anyways, if you have any further problems/questions, please leave a comment below and I'll see what I can do. Hope I helped :P

-Dyler3