im making a simulator and a part of the script wont work?

its coming up with a error message saying " 20:50:56.931 - Players.cjkizzy286.Backpack.Milk.Script:6: attempt to perform arithmetic (add) on string and number" but if i change the value from a string to a int or number nothing happens the script wont run there are 2 bits of code

local data = game:GetService("DataStoreService")
local datastore = data:GetDataStore("Milk")-- name of it
local datastore = data:GetDataStore("Coins")-- name of it
local rebirth = game.Workspace.Values.Rebirthmultiplier

game.Players.PlayerAdded:Connect(function(Player)
	local Leaderstats = Instance.new("Folder")
	Leaderstats.Name = "leaderstats"
	Leaderstats.Parent = Player
	
	local Milk = Instance.new("StringValue",Leaderstats)
	Milk.Name = "Milk"
	Milk.Value = 0
	
	local Coins = Instance.new("StringValue",Leaderstats)
	Coins.Name = "Coins"
	Coins.Value = 0
	
	function Call()
	local sell = Milk.Value
	Coins.Value = Coins.Value + Milk.Value
	Milk.Value = 0
end

end)

game.Players.PlayerRemoving:Connect(function(player)
	datastore:SetAsync(player.UserId, player.leaderstats.Milk.Value)
	datastore:SetAsync(player.UserId, player.leaderstats.Coins.Value)
end)

game.Workspace.Sell.Sellpart.Touched:Connect(function(touched)
	Call()
end)

and

local Leaderstats = Instance.new("Folder")
local Milk = Instance.new("StringValue",Leaderstats)
local Coins = Instance.new("StringValue",Leaderstats)

script.Parent.Activated:Connect(function()
	Milk.Value = Milk.Value + 1
end)