Another 'attempt to index a nil value', please help?

-- Zombie Controller
-- by alphagamer150

-- Get services
local PathfindingService = game:GetService("PathfindingService")
local Players = game:GetService("Players")

-- Create varibles
local Zombie = script.Parent

-- Main Loop
-- while not Python:
while wait() do
	local PlayerList = Players:GetPlayers()
	local LowestDistance = 0
	local ClosestPlayer
	for i = 1, #PlayerList do
		local PlayerPos = game.Workspace:FindFirstChild(PlayerList[i].Name).HumanoidRootPart.Position
		local ZombiePos = Zombie.HumanoidRootPart.Position
		print(PlayerPos)
		if (PlayerPos - ZombiePos).magnitude < LowestDistance then
			print("Test")
		end
	end
end

Script above keeps giving me 19:36:12.091 - Workspace.Zombie.Script:18: attempt to index a nil value

Their character may not have spawned yet, causing FindFirstChild to return nil. Additionally, I wouldn't recommend searching for characters using the username in the Workspace - imagine someone named Model joining your game and causing weird things to happen. Instead, I'd recommend using the Player.Character property. If they don't have one, use Player.CharacterAdded to wait for it. Here's an example.

local player = PlayerList[i]
local character = player.Character or player.CharacterAdded:Wait()
local rootPartPos = character:WaitForChild('HumanoidRootPart').Position

-- ...

Although, if you're yielding like this (which may pause the script for whatever amount of time), I'd recommend using spawn or coroutines to run this in parallel rather than one at a time. Advice beyond that is outside the scope of this answer, but yeah. Your issue is that either the character or HumanoidRootPart don't exist. Since it's most likely the former, here.