Choosing a player?

That just means that the 2nd argument in the math.random function is smaller than the 1st argument. Basically, the script is running when there are no players in the game.

To fix this, simply add a conditional that checks to see if the game has at least 2 players, to prevent errors, like so:

local playerstorage = game:GetService("Players")
local players = {}

while true do
	for _, player in pairs(game.Players:GetPlayers()) do
		if player and player.Character then
			local humanoid = player.Character:WaitForChild("Humanoid")
			if humanoid and humanoid.Health > 0 then
				table.insert(players, player)
			end
		end
	end
	if #players >= 2 then
		local alien = players[math.random(1, #players)]
		--More code
	end
	wait(0.1)
end

Hope this helped!

More info: math.random