Help targetting players via StarterGui?

It's probably because you're not waiting for your character to load. In a LocalScript, in your StarterGui, you could use something like this:

local player = game:service('Players').LocalPlayer
local char = player:WaitForChild("Character")
local humanoid = char:WaitForChild("Humanoid")

if player then
    print("Player found")
end

if char then
    print("Character found")
end

if humanoid then
    print("Humanoid found")
end

LocalScripts are handy this way, but :WaitForChild() is the key here.

How about instead of;

Humanoid = script.Parent.Parent.Parent.Parent.Character.Humanoid

Try;

repeat wait(0)until game:FindService("Players")and game.Players.LocalPlayer and game.Players.LocalPlayer.Character and game.Players.LocalPlayer.Character:FindFirstChild("Humanoid") --Repeats waiting until finds Players, LocalPlayer, LocalPlayer's Character, and LocalPlayer's Characrer's Humanoid
local Plr = game.Players.LocalPlayer --LocalPlayer
local Char = Plr.Character --LocalPlayer's Character
local Humanoid = Char.Humanoid --LocalPlayer's Character's Humanoid

--Rest of Coding here

Hope this helped!

--No need for .Character, just Parent.Humanoid 

So something like this would work?

local Blood = script.Parent
local Humanoid = script.Parent.Parent.Parent.Parent.Character.Humanoid
local player = game:service('Players').LocalPlayer
local char = player:WaitForChild("Character")
local humanoid = char:WaitForChild("Humanoid")

if player then
    print("Player found")
end

if char then
    print("Character found")
end

if humanoid then
    print("Humanoid found")
end


if Humanoid.Health == 100 then
    Blood.Visible = false
elseif Humanoid.Health >= 80 < 100 then
    Blood.Visible = true
    Blood.BackgroundTransparency = 0.2
elseif Humanoid.Health >= 60 < 80 then
    Blood.Visible = true
    Blood.BackgroundTransparency = 0.4
elseif Humanoid.Health >= 40 < 60 then
    Blood.Visible = true
    Blood.BackgroundTransparency = 0.6
elseif Humanoid.Health >= 0 < 40 then
    Blood.Visible = true
    Blood.BackgroundTransparency = 0.8
else
    print("An error has occured with the Blood Gui script.")

end