Why won't this shift to sprint script function?

First, the error. I believe that the line 3 Player.CharacterAdded:Wait() is kind of breaking your script because it yields your script until character is actually added. What I mean is that if your character is already loaded, the CharacterAddedevent won`t fire and that script will have to wait forever. To fix that, you can do this so it will check if character is loaded, and if not it will wait.

local character = Player.Character or Player.CharacterAdded:Wait()

Also, I think you should use ContextActionService instead of UserInputService because it is making you code a bit more complicated than it should be. To use ContextActionService, you can do

local function sprint(actionName, inputState, inputObject)
    if (inputState == Enum.UserInputState.Begin) then
        --Your Sprint Code
    elseif (inputState == Enum.UserInputState.End) then
        --Make your speed to normal speed
    end
end

game:GetService("ContextActionService"):BindAction("Sprint"--[[Name of action--]],sprint--[[function--]],false--[[if you want to make a mobile button for this action, make it true--]],Enum.KeyCode.LeftShift--[[when to fire--]]) 

I hope this helps :D

you can have this script

function onPlayerEntered(player)
    repeat wait () until player.Character ~= nil
    local s = script.SprintScript:clone()
    s.Parent = player.Character
    s.Disabled = false
    player.CharacterAdded:connect(function (char)
        local s = script.SprintScript:clone()
        s.Parent = char
        s.Disabled = false      
    end)
end

game.Players.PlayerAdded:connect(onPlayerEntered)

then do a `local script`

local mouse = game.Players.LocalPlayer:GetMouse()
local running = false

function getTool()  
    for _, kid in ipairs(script.Parent:GetChildren()) do
        if kid.className == "Tool" then return kid end
    end
    return nil
end

then do a local script and put it inside the script

mouse.KeyDown:connect(function (key) -- Run function
    key = string.lower(key)
    if string.byte(key) == 48 then
        running = true
        local keyConnection = mouse.KeyUp:connect(function (key)
            if string.byte(key) == 48 then
                running = false
            end
        end)
        for i = 1,5 do
            game.Workspace.CurrentCamera.FieldOfView = (70+(i*2))
            wait()
        end
        game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 30
        repeat wait () until running == false
        keyConnection:disconnect()
        game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 16
        for i = 1,5 do
            game.Workspace.CurrentCamera.FieldOfView = (80-(i*2))
            wait()
        end
    end
end)