Does anybody know why FindFirstChild is indexing as nil?

local PoBox = game.Workspace.Assets.PostBox31
local Posting = game.Players.LocalPlayer:FindFirstChild("Posting")
Posting.Value = false

PoBox.ClickDetector.MouseClick:Connect(function(clicked)
	if Posting.Value == true then
		game.ReplicatedStorage.AlreadyPosting:FireClient()
		print("Already posted fired")
	else
		game.ReplicatedStorage.PostingStarted:FireClient()
		print("Fired")
	end
end)

Output:
17:07:25.565 - Workspace.Assets.PostBox31.PostingScript:2: attempt to index nil with 'FindFirstChild' 17:07:25.566 - Stack Begin 17:07:25.566 - Script 'Workspace.Assets.PostBox31.PostingScript', Line 2 17:07:25.567 - Stack End

Using WaitForChild shows the error: 17:18:35.169 - Workspace.Assets.PostBox31.PostingScript:2: attempt to index nil with 'WaitForChild'

Also if that doesnt help then you can also define posting in the mouseclick function

local PoBox = game.Workspace.Assets.PostBox31

PoBox.ClickDetector.MouseClick:Connect(function(clicked)
local Posting = game.Players.LocalPlayer.Posting
Posting.Value == not Posting.Value 
  if Posting.Value == true then
        game.ReplicatedStorage.AlreadyPosting:FireClient()
        print("Already posted fired")
    else
        game.ReplicatedStorage.PostingStarted:FireClient()
        print("Fired")
    end
end)

It is probably because you are not doing it in a local script, so that is why the player is nil, you can access local player with Local Script only.