???

[crisp aspen]

local UserInputService = game:GetService("UserInputService")

local player = Players.LocalPlayer
local character = player.Character or player.CharacterAdded:wait()

local function onInputStarted(inputObject, processedEvent)
    if processedEvent then return end
    
    if inputObject.KeyCode == Enum.KeyCode.LeftShift then
        local character = Players.LocalPlayer.Character
        if not character then return end
        
        local humanoid = character:FindFirstChildOfClass("Humanoid")
        if not humanoid then return end
        
        humanoid.WalkSpeed = 20
        
        local humanoid = character:WaitForChild("Humanoid")
        local animator = humanoid:WaitForChild("Animator")
        
        local runAnimation = Instance.new("Animation")
        runAnimation.AnimationId = "rbxassetid://119708205580016"
        
        local runAnimationTrack = animator:LoadAnimation(runAnimation)
        
        runAnimationTrack:Play()
    end
end

local function onInputEnded(inputObject, processedEvent)
    if processedEvent then return end

    if inputObject.KeyCode == Enum.KeyCode.LeftShift then
        local character = Players.LocalPlayer.Character
        if not character then return end

        local humanoid = character:FindFirstChildOfClass("Humanoid")
        if not humanoid then return end

        humanoid.WalkSpeed = 12
        
        local runAnimation = Instance.new("Animation")
        runAnimation.AnimationId = "rbxassetid://119708205580016"
        
        local runAnimationTrack = animator:LoadAnimation(runAnimation)
        
        runAnimationTrack:Stop()
    end
end

UserInputService.InputBegan:Connect(onInputStarted)

UserInputService.InputEnded:Connect(onInputEnded)```

[near gust]

What's wrong with your script

Give me an output error

I think when you're release a shift button, the humanoid walkspeed should be set by 16 instead of 12

[tawny crater]

You forgot to get the animator in the inputEnded function