What @MEsAv2 said is right, and judging by your code above is probably what the issue is - try this
local function playerAdded(player)
local attributes = Instance.new("Folder")
attributes.Name = "Attributes"
attributes.Parent = player
local attack = Instance.new("IntValue")
attack.Name = "Attack"
attack.Value = 5
attack.Parent = attributes
end
--Incase player joined earlier than this script ran
for _, player in ipairs(Players:GetPlayers()) do
task.spawn(playerAdded, player)
end
Players.PlayerAdded:Connect(function(playerAdded)
(Also as a side note you should not do Instance.new(“Folder”, player) if you are going to be changing attributes of the new instance as it is not optimised)