I keep on getting an error when trying to detect the humanoidrootpart moving. why is it doing this?

I'm trying to make a billboard gui show up when you get near a specific npc, however, i cant even figure out how to detect when the player moves. I can't seem to figure out what i'm doing wrong, the error is

Players.Galaxybombboy.PlayerGui.LocalScript:7: attempt to index nil with 'Changed'

and the script is

local player = game.Players.LocalPlayer
local character = player.Character or player.CharacterAdded:Wait()
local rockman = workspace["Rock Man - Shop Keeper"]
local rockhumroot = rockman:FindFirstChild("HumanoidRootPart")


character:FindFirstChild("HumanoidRootPart").Changed:Connect(function()
	if (character:FindFirstChild("HumanoidRootPart").Position - rockhumroot.Position).Magnitude < 50 then
		print("working")
	end
end)

What am i doing wrong?

You Cant Cause When Your In Game And You Walk And When You open Your Characters Humanoid Root Part Properties The Position Wont Change So It Would be Better If You use Head That Would Work :D

Code Replaced By Head:

local player = game.Players.LocalPlayer
local character = player.Character or player.CharacterAdded:Wait()
local rockman = workspace["Rock Man - Shop Keeper"]
local rockhumroot = rockman:FindFirstChild("Head")


character:FindFirstChild("Head"):GetPropertyChangedSignal("Position"):Connect(function()
    if (character:FindFirstChild("Head").Position - rockhumroot.Position).Magnitude < 50 then
        print("working")
    end
end)

You should try changing

character:FindFirstChild("HumanoidRootPart")

to

character:WaitForChild("HumanoidRootPart")

Let me know if this works!