I tried adding a datastore to the leaderboard and it doenst work?

Improvements

Use :GetService() to retrieve the Players service

Use :WaitForChild() to make sure an instance exists before using it

Set the Parent of a new instance on a separate line, it is more efficient for the API

Issues

"data" is not predefined before the pcall

Lua is case sensitive, so "GetaSync" would not be called correctly, it must be GetAsync

Revised Server Script

local Players = game:GetService("Players")
local datastoreservice = game:GetService("DataStoreService")
local mydata = datastoreservice:GetDataStore("mydata")

Players.PlayerAdded:Connect(function(player)
	local leaderstats = Instance.new("Folder")
	leaderstats.Name = "leaderstats"
	leaderstats.Parent = player
	
	local HatPoints = Instance.new("IntValue")
	HatPoints.Name = "HatPoints"
	HatPoints.Parent = leaderstats
	
	local Credits = Instance.new("IntValue")
	Credits.Name = "Credits"
	Credits.Parent = leaderstats
	
	local PlayerUserid = "Player_"..player.UserId
	
	local data = nil
	local success, errormessage = pcall(function()
		data = mydata:GetAsync(PlayerUserid)
	end)
	
	if success then
		Credits.Value = data
	else
		warn(errormessage)
	end
end)

Players.PlayerRemoving:Connect(function(player)
	local PlayerUserid = "Player_"..player.UserId
	local data = player:WaitForChild("leaderstats"):WaitForChild("Credits").Value
	
	local success, errormessage = pcall(function()
		mydata:SetAsync(PlayerUserid, data)
	end)
	
	if not success then
		warn(errormessage)
	end
end)