Animation won't play when shift is pressed?

Hello. Try using UserInputService as Mouse.KeyDown is deprecated. Try this:

local UserInputService = game:GetService("UserInputService")

local anim = Instance.new("Animation")
anim.AnimationId = "rbxassetid://2510238627"

local plr = game:GetService("Players").LocalPlayer
local mouse = plr:GetMouse()

local char = plr.Character or plr.CharacterAdded:Wait()
local human = char:WaitForChild("Humanoid")

local track = human:LoadAnimation(anim)
track.Priority = Enum.AnimationPriority.Action

UserInputService.InputBegan:Connect(function(input, gameProcessedEvent)
	if input.KeyCode == Enum.KeyCode.LeftShift and not gameProcessedEvent then
		track:Play()
		human.WalkSpeed = 23
	end
end)

UserInputService.InputEnded:Connect(function(input, gameProcessedEvent)
	if input.KeyCode == Enum.KeyCode.LeftShift and not gameProcessedEvent then
	       track:Stop()
		human.WalkSpeed = 16
	end
end)

The InputBegan function fires when an input is detected. Then there's an if statement checking if the input is left shift and that the player isn't in chat. Then it plays the animation and changes the walkspeed.