How do I create a custom sprint script, as well as change the FOV so that it is higher?

I'm working on a game for a friend, and he asked me to make a sprint script. As I didn't want to use an already made one, I attempted to make one myself, which doesn't shoot any errors when I use it, but it does not change the speed. I do not know how to change the FOV either. Please help!

UIS = game:GetService("UserInputService")

function sprint()
	local debounce = 1
	if debounce then
		--make fov
		debounce = 0
		game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 24
		wait(0.1)
		debounce = 1
	end
end

function stopsprint()
	game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 16
end

game.Players.PlayerAdded:Connect(function(plr)
	UIS.InputBegan:Connect(function(input)
		if input.UserInputType == Enum.KeyCode.LeftShift or input.UserInputType == Enum.KeyCode.RightShift then
			sprint()
		end
	end)
	UIS.InputEnded:Connect(function(input)
		if input.UserInputType == Enum.KeyCode.LeftShift or input.UserInputType == Enum.KeyCode.RightShift then
			stopsprint()
		end
	end)
end)

I'd use TweenService with the FOV changer as it looks smoother than just changing the camera's FOV. Make sure it's a LocalScript inside StarterPlayer > StarterCharacterScripts. Try this:

local UserInputService = game:GetService("UserInputService")
local Workspace = game:GetService("Workspace")
local TweenService = game:GetService("TweenService")
local Players = game:GetService("Players")
local localPlayer = Players.LocalPlayer
local humanoid = localPlayer.Character:WaitForChild("Humanoid")
local defaultFOV = 70

local properties1 = {FieldOfView = defaultFOV + 4}

local info1 = TweenInfo.new(
	0.25, 
	Enum.EasingStyle.Bounce,
	Enum.EasingDirection.InOut
) 

local tween1 = TweenService:Create(Workspace.CurrentCamera, info1, properties1)

local properties2 = {FieldOfView = defaultFOV}

local info2 = TweenInfo.new(
	0.25, 
	Enum.EasingStyle.Sine,
	Enum.EasingDirection.InOut
) 

local tween2 = TweenService:Create(Workspace.CurrentCamera, info2, properties2)

UserInputService.InputBegan:Connect(function(input, gameProcessedEvent)
	if not gameProcessedEvent and input.KeyCode == Enum.KeyCode.LeftShift then
		humanoid.WalkSpeed = 24
		tween1:Play()
	end
end)

UserInputService.InputEnded:Connect(function(input, gameProcessedEvent)
	if not gameProcessedEvent and input.KeyCode == Enum.KeyCode.LeftShift then
		humanoid.WalkSpeed = 16
		tween2:Play()
	end
end)

TweenService can be a bit hard to explain, so I'll leave the Roblox Developer Hub link for you.