DataStore won't save for some reason?

The code below will not get the data store. Your code will throw an error as DataStoreService does not have a function called GetAsync or SetAsync.

local dataStore = game:GetService("DataStoreService") -- this is the service not the data store object
dataStore:GetDataStore("Cash")  -- this gets the cash data store but it is never used

The second problem is with the line. This is not assign the variables the values passed back from pcall. Use the assignment operator =

local success,err pcall(function()

Lastly you should not be be connecting a new PlayerRemoving event each time a player joins. This will stack causing your saves to fail.

game.Players.PlayerAdded:Connect(function(plr)
	
	-- this adds a new function to the event each time
	game.Players.PlayerRemoving:Connect(function(plr)
	end)
end)

Putting this all together

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

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

    local cash = Instance.new("IntValue")
    cash.Name = "Cash"

    cash.Parent = leaderstats
	leaderstats.Parent = plr -- parent to game last
	
    local success, result = pcall(function()
        return dataStore:GetAsync(plr.UserId) -- return the value from the function call
    end)

    if success then
        cash.Value = result
    else -- only else is needed
        warn(result) -- result will hold the error message if there is one
        print("when plr joining saving data didnt work")
    end

end)

game.Players.PlayerRemoving:Connect(function(plr)
	local success,result = pcall(function()
		dataStore:SetAsync(plr.UserId, plr.leaderstats.Cash.Value)
	end)
	
	if success then
	    print("Worked saving")
	else
	    warn(result)
	    print("something went wrong saving data")
	end
end)

Other notes

You should include a retry if one save/load fails.

Hope this helps. Please comment if you have any other questions about this code.