attempt to perform arithmetic (mul) on number and nil?

Firstly, game.Players.PlayerAdded only accepts 1 parameter, which is the new player. Secondly, you can't manipulate the GUI on the server, you have to do it on the client. Thirdly, you can send tuple arguments with the FireServer() function, however, the first parameter has to be the player. So what I suggest you do is on the client, you change the playerGui that way, and on the server, you change the leaderstats only.

Local Script

local player = game.Players.LocalPlayer
local playerGui = player:WaitForChild("PlayerGui")
script.Parent.MouseButton1Click:Connect(function() --connect is deprecated. Use Connect.
	local amount = script.Parent.Parent.Parent.TextBox.Text
	amount = tonumber(amount) --This is VERY vulnerable, always change the values on the server. Exploiters can exploit it.
	if player.leaderstats.Money.Value  >= 1000000 * amount then --This is only reading the value, which is fine on the client.
		game.ReplicatedStorage.rebirth:FireServer(amount)
	else
		player.PlayerGui.Gui.badframe.Visible = true
		wait(1)
		player.PlayerGui.Gui.badframe.Visible = false
	end
end)

Server Script

game.ReplicatedStorage.rebirth.OnServerEvent:Connect(function(player, amount)
	local leaderstats = player:WaitForChild("leaderstats") --This is a better alternative since you need it.
	leaderstats.Rebirths.Value = leaderstats.Rebirths.Value + amount
	leaderstats.Money.Value = 0
	player.Character.Head:Destroy()
end)

(Note on the amount variable. Always change the values on the server. You can use an IntValue and place it ReplicatedStorage. But that's kind of off-topic.)