Shift to Sprint script not working, need assistance; If possible?

Made this awhile back and want to get started at making my own game. Idky my sprinting script aint working, but the gui is working which is great. If you can help, it would be greatly appreciated.

local CAS = game:GetService("ContextActionService")
local RE = game:GetService("ReplicatedStorage"):WaitForChild("RE")
local fill = script.Parent:WaitForChild("top"):WaitForChild("fill")

local stam = 50
local speed = 16
local sprinting = false

local function sprint (str,state,object)
	if stam <1 then return end
	speed = state == Enum.UserInputState.Begin and 24 or 16
	sprinting = state == Enum.UserInputState.Begin
	while sprinting and stam > 0 and wait(.1) do
		stam = stam - 1
		fill:TweenSize(UDim2.new(stam/50, 0, 1, 0),"Out","Quint", .1, true)  
		RE:FireServer('sprint', (speed))
	end
 	sprinting = false
	speed = 16
	RE:FireServer('sprint', (speed))
	wait(1)
	while not sprinting and stam < 50 and wait (.5) do
		fill:TweenSize(UDim2.new(stam/50, 0, 1, 0), "Out", "Quint", .1, true)
		stam = stam +0.5
	end
end

CAS:BindAction("sprint", sprint, true, Enum.KeyCode.LeftShift)

--sprinting does not work, but the gui is currently working when you hold down, and it also regens back.

hey man, i dont know to much of really advanced scripting but i recently made a sprint script with a gui as well and ill link that to you here to look through:

https://www.roblox.com/library/3455933485/SprintGUI

alternatively here is the script for it:

--variables
local textlabel = script.Parent
local player = game.Players.localPlayer
local stamina = 100
local uis = game:GetService("UserInputService")
local isSprinting = false



uis.InputBegan:Connect(function(key)
	if key.KeyCode == Enum.KeyCode.LeftShift and stamina >0 then
		isSprinting = true
		player.Character.Humanoid.WalkSpeed = 25 
		while key.KeyCode == Enum.KeyCode.LeftShift and stamina >0 and isSprinting == true do
			stamina = stamina -1
			print(stamina)
			wait(0.1)
			if stamina <= 0 then
				isSprinting = false
				player.Character.Humanoid.WalkSpeed = 16
			end
		end
		wait(0.1)
	end
end)





	uis.InputEnded:Connect(function(key)
		if key.KeyCode == Enum.KeyCode.LeftShift then
			isSprinting = false
		
		player.Character.Humanoid.WalkSpeed = 16
		wait(2)
			while key.KeyCode == Enum.KeyCode.LeftShift and stamina <100 and isSprinting == false do
			stamina = stamina +1 
			wait(0.1)
			print(stamina)
			end
		end
	end)
	while wait (0.1) do
		textlabel.Text = ("Sprint: "..stamina)
	end