My leaderstats save script doesnt work! OUTPUT error in desc. Help?

When the player first joins the game, they don't have any value stored in CashDataStore. Therefore, calling CashDataStore:GetAsync(playerUserId) returns nil. Meaning that, at Cash.Value = data.Cash, you're actually saying Cash.Value = nil.Cash. To fix this, you have to check if the datastore is nil and if so, set it to a starter value. That can be done this way:

data = CashDataStore:GetAsync(playerUserId) or {["Cash"] = 0}

When you write variable = value1 or value2, you're setting the value of variable to value1, unless value1 is nil, in which case you set the value to value2.

Testing your script, I also found an error when you try to save everything at this line:

CashDataStore:SetAysnc(playerUserId, data)

The only issue here though is a typo at SetAsync, you wrote SetAysnc. This is easily fixable:

CashDataStore:SetAsync(playerUserId, data)

Here is the new script in its entirety:

local DataStoreService = game:GetService("DataStoreService")
local CashDataStore = DataStoreService:GetDataStore("CashDataStore")

game.Players.PlayerAdded:Connect(function(player)
    local leaderstats = Instance.new("Folder")
    leaderstats.Name = "leaderstats"
    leaderstats.Parent = player

    local Cash = Instance.new("IntValue")
    Cash.Name = "Cash"
    Cash.Parent = leaderstats

    local playerUserId = "Player_"..player.UserId
   
	--// load data
    local data
    local success, errormessage = pcall(function()
         data = CashDataStore:GetAsync(playerUserId) or {["Cash"] = 10}
    end)

    if success then
		Cash.Value = data.Cash
	end
end)


game.Players.PlayerRemoving:Connect(function(player)
    local playerUserId = "Player_"..player.UserId
    local data = {["Cash"] = player.leaderstats.Cash.Value}
   
 	local success , errormessage = pcall(function()
		CashDataStore:SetAsync(playerUserId, data)
	end)

    if success then 
  		print("DataSaved")
 	else
		print("An Error Occured Saving Data")
      	warn(errormessage)
   	end
end)

However, I do wanna say this; I would heavily discourage you from using PlayerRemoving to save everything as the function is extremely unreliable. Instead, I would recommend you to either save the data every time something changed, like this:

Cash.Changed:Connect(function()
	data.Cash = Cash.Value

	local success , errormessage = pcall(function()
		CashDataStore:SetAsync(playerUserId, data)
	end)
end)

or create a loop that saves the date every now and then, like this:

while wait(60) do -- Make sure to include a long wait time, as changing a datastore is heavily demanding
	data.Cash = Cash.Value

	local success , errormessage = pcall(function()
		CashDataStore:SetAsync(playerUserId, data)
	end)
end 

If I missed anything, or you have any further questions, feel free to ask.