LocalScript works in Studio but not in Player Mode. What can be done to make it work?

Update: I have figured out what seems to be the problem, although I don't know how I should go about fixing it. It appears that when referencing back to humanoid, the script results in an error in player mode.

SOLUTION: Replacing the first line with local humanoid = workspace:WaitForChild(game.Players.LocalPlayer.Name):WaitForChild("Humanoid") did the trick. If you have a better or more effective solution, feel free to answer.

I wrote a script that alters a characters' walkspeed when left shift is pressed and jumping power when the space button is pressed. All of the information regarding ... Stamina.Value works as desired. The problem is that this script works in studio but not in player mode. For instance, when I press the respective keys in studio, the script alters the walkspeed and jumping power. However, this is not the case in player mode. In player mode, the walkspeed and jumping power does not change.

This script is a local script placed within another local script that is placed within StarterGui. The layout looks like this:

• StarterGui

  • ScreenGUI

    • Frame

      • Local Script 1

        • IntValue, titled "Stamina"

        • Local Script 2 (the script I am referring to)

I don't understand what the issue is because I have a similar script to this one, except it alters the stamina value of an IntValue and it works as desired in both studio and player mode. Yet, this script does not want to make my life easy. See below:

--Indexing variables [This works].
local humanoid = game.Players.LocalPlayer.Character:FindFirstChild("Humanoid")
local input = game:GetService("UserInputService")
local leftShift, space = 0, 0
local t, f = true, false

--Using InputBegan and InputEnd to figure out whether the keys are pressed or not. [This works].
input.InputBegan:Connect(function(key)
    if (key.KeyCode == Enum.KeyCode.LeftShift) then
        leftShift = t
    elseif (key.KeyCode == Enum.KeyCode.Space) then
        space = t
    end
end)

input.InputEnded:Connect(function(key)
    if (key.KeyCode == Enum.KeyCode.LeftShift) then
        leftShift = f
    elseif (key.KeyCode == Enum.KeyCode.Space) then
        space = f
    end
end)

--Running a function that executes code depending on whether a key is pressed or not. [Does not work].
function walkSpeedProc() -- Executes well in studio but not in player mode. 
    if leftShift == t and script.Parent.Stamina.Value < 6 then
        humanoid.WalkSpeed = 16
        humanoid.JumpPower = 0
    elseif leftShift == t and script.Parent.Stamina.Value >= 6 then
        humanoid.WalkSpeed = 25
    elseif leftShift == f then
        humanoid.WalkSpeed = 16
    end

    if space == t and script.Parent.Stamina.Value >= 7 then
        humanoid.JumpPower = 50
    elseif space ==  t and script.Parent.Stamina.Value < 7 then
        humanoid.JumpPower = 0
    elseif space == f then
        humanoid.JumpPower = 0
    end
end

--Loops the function written previously to check repeatedly for key presses and what not. [Does not work].
while true do
    wait()
    walkSpeedProc()
end

If more information is needed, please let me know and I will provide you with the extra information.