It is because you use game.Players.LocalPlayer
. You cannot use that on the server. Instead do:
script.Parent.Touched:connect(function(Hit)
local Player = game:GetService("Players"):GetPlayerFromCharacter(Hit.Parent)
if Player then
local Points = Player:WaitForChild("leaderstats"):WaitForChild("Pong")
Points.Value = Points.Value + 15
end
end)