Sprint animation script help fix please?

I want to play an animation playing when Shift is being hold, and when you release it. The animation stops. How do I add it in here, and also the script only works in studio, but not in roblox client. z_z Help fix! Take a look below.

if script.Parent.Name == "PlayerGui" then
    local mouse = game.Players.LocalPlayer:GetMouse()
    local running = false
    local gui = script.sprntGui.Frame
    energy = 100
    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() 
                if energy ~= 20 then
                    gui.Size = gui.Size-UDim2.new(0,5,0,0) energy = energy-1
                else
                    running = false
                end
            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
            coroutine.resume(coroutine.create(function()
                repeat 
                    wait() if energy ~= 100 then gui.Size = gui.Size+UDim2.new(0,5,0,0) energy = energy+1 end
                until
                    running == true
            end))
        end
    end)
else
    print("YOU MUST PUT THIS INSIDE STARTGUI")
end