How do I fix this Issue with a datastore (invalid argument #3 (string expected, got nil)?

When I Join the game it keeps say this error
ServerScriptService.DataStorage.RankCreator:36: invalid argument #3 (string expected, got nil):

The error is occuring at line 36

local DataStoreService = game:GetService("DataStoreService")
local dataStore = DataStoreService:GetDataStore("DataStore")

local function saveData(plr)

	local dataTable = plr.leaderstats("Rank").Value

	local success, err = pcall(function()
		dataStore:SetAsync(plr.UserId, dataTable) -- Save the data with the player UserId, and the table we wanna save
	end)

	if success then -- If the data has been saved
		print("Data has been saved!")
	else -- Else if the save failed
		print("Data hasn't been saved!")
		warn(err)		
	end
end

game.Players.PlayerAdded:Connect(function(plr)
	local leaderstats = Instance.new("Folder")
	leaderstats.Parent = plr
	leaderstats.Name = "leaderstats"
	
	local rank = Instance.new("StringValue")
	rank.Parent = leaderstats
	rank.Name = "Rank"
	
	local data -- We will define the data here so we can use it later, this data is the table we saved
	local success, err = pcall(function()
		data = dataStore:GetAsync(plr.UserId) -- Get the data from the datastore
		print("getting data...")
	end)
	
	if success then
		rank.Value = data
	else 
		print("The player has no data!")
	end
	
	rank.Changed:Connect(function()
		print("rank changed")
	end)
	
end)

game.Players.PlayerRemoving:Connect(function(player) -- When a player leaves
	local success, err  = pcall(function()
		saveData(player) -- Save the data
	end)

	if success then
		print("Data has been saved")
	else
		print("Data has not been saved!")
	end
end)

You didn't assign any value to data when you define it at Line 29 that's why it returned nil. rank.Value at Line 36 has to be string. Hope this helps!

Success will be true even if there is no data. You will have to check if data exists as well before setting rank.Value.

The other answers to this question are indeed telling you the mistake you made, but it does not look like they are being quite clear enough on how exactly to apply the fix. In short, all you need at the very least is an and operator. However, I am going to go a little more in depth than that because I feel that this question stems from a lack of understanding of datastores. Let's go over this.

local data
local success, err = pcall(function()
	data = dataStore:GetAsync(plr.UserId)
	print("getting data...")
end)

if success then
	rank.Value = data
else 
	print("The player has no data!")
end

When making a datastore call like the above, the 'success' variable just indicates if the call made it over the network. So, you could ask for information from a datastore and you may find that the call is successful, but it doesn't mean that there is actually any data saved.

To elaborate on the why a datastore request might fail sometimes (and you might already know this!), when you ask for something over the network such as datastore information, that request has to make it over to Roblox's servers and hence could fail at times, usually because of a bad connection. The success variable tells you if it failed or not and that's its job. Again, it doesn't actually indicate that there is any saved data.

What you want to do is check if the call was successful AND if data was returned, and only then update your value object with the returned data.

if success and data then
	rank.Value = data
else
	print("The player has no data OR the datastore request failed!")
end

That's the fix. Very simple. Now, if you wish to handle datastore call failure in some way (retrying the request some number of times, etc.) you could take this a step further by firstly checking if the call was successful, then checking for the existence of saved data, like so:

if success then
	print("The datastore request was successful.")

	if data then
		print("Saved data found")
	else
		print("No saved data found")
	end
else
	print("The datastore request failed: "..err") -- Print the error message to see why the call failed if you're curious!
end