Your leaderstats script works for me. One thing though, you shouldn’t use the second parameter of Instance.new but instead assign the parent after.
Code without the second parameter:
local players = game:GetService("Players")
players.PlayerAdded:connect(function(player)
local leaderstats = Instance.new("Folder")
leaderstats.Name = 'leaderstats'
leaderstats.Parent = player
local Yen = Instance.new("NumberValue")
Yen.Name = "Yen"
Yen.Value = 0
Yen.Parent = player
local Rank = Instance.new("StringValue")
Rank.Name = "Rank"
Rank.Value = "Mizunoto"
Rank.Parent = leaderstats
end)