Help with Knife Animations?

You have to load the animation into the humanoid:

local tool = script.Parent
local player = game:GetService("Players").LocalPlayer
hum=player.Character:WaitForChild("Humanoid")

tool.Equipped:connect(function(mouse) 
    mouse.Button1Down:connect(function()
        local animTrack = hum:LoadAnimation( script.Parent.LeftSlash)
        animTrack:Play()
    end)
end)

Well, you have to play the animation inside of the humanoid, not the tool. Therefore, I suggest putting the animation inside of ServerStorage (or if you don't prefer that, you could go the classic way and use Lighting) . First, set the variables:

local animation = game.Lighting.LeftSlash:Clone() --[[ This clones the animation so that it does not directly move the one inside ServerStorage and load it into the humanoid, allowing for it to be used multiple times.]]
local LoadAnimation = player.Character.Humanoid:LoadAnimation(animation) --[[ This loads the animation into the humanoid, but does not play it.]]
LoadAnimation:Play() --[[This plays the loaded animation. To stop it you would do LoadAnimation:Stop() as long as it's inside the same area where the animation was ran.]]

That leaves us with this:

local tool = script.Parent
local player = game:GetService("Players").LocalPlayer

tool.Equipped:connect(function(mouse) 
    mouse.Button1Down:connect(function()
    local animation = game.Lighting.Whip:Clone()
    local LoadAnimation = player.Character.Humanoid:LoadAnimation(animation)
    LoadAnimation:Play()
--If you want to stop it within a certain amount of time
    wait(3)
    LoadAnimation:Stop()
    end)
end)

If anything doesn't work, comment what the error is and I'll see if I can fix it. I'm not the best with animations but I believe this should work.