Why does this chat message script not work in a server but works in studio mode?

This is my script, it works in studio but doesn't work in studio. Please help.

function Join(plr)

game.StarterGui:SetCore("ChatMakeSystemMessage", {
Text = "[SERVER] :: "..plr.Name.." Joined the server!";
Color = Color3.fromRGB(220, 180, 58); 
Font = Enum.Font.ArialBold;
FontSize = Enum.FontSize.Size18;
})
end
game.Players.PlayerAdded:connect(Join)

function Leave(plr)
game.StarterGui:SetCore("ChatMakeSystemMessage", {
Text = "[SERVER] :: "..plr.Name.." Left the server!";
Color = Color3.fromRGB(220, 180, 58); 
Font = Enum.Font.ArialBold;
FontSize = Enum.FontSize.Size18;
})
end
game.Players.PlayerRemoving:connect(Leave)