Triggering Animation By Pressing A Key Doesnt Work?

local player = game.Players.LocalPlayer
repeat wait() until player.Character.Humanoid
local humanoid = player.Character.Humanoid
local mouse = player:GetMouse()

local anim = Instance.new("Animation")
anim.AnimationId = "http://www.roblox.com/asset/?id=" -- id of animation

mouse.KeyDown:connect(function(key)
	if key == "l" then 
		local playAnim = humanoid:LoadAnimation(anim)
		playAnim:Play()
	end
end)

#Client vs Server Sided Local scripts only affect things the user sees (like GUIs, camera or local parts). Server sided scripts can change things in the server, like the animation a player does or the CFrame of a part, for example. Local scripts can't change the server or affect other users, and server sided scripts are only used for... the server. You can use Remote functions/events to let the client and server communicate with each other.

#Remote Events We're using remote events since we just want the animation to play. We will need 1 local script to record the input and 1 server sided script to play the animation.

--Remote Event is placed inside the server script in ServerScriptService
script.RemoteEvent.OnServerEvent(function(player) --When the client fires the server...
	local humanoid = player.Character:FindFirstChild("Humanoid")
	if humanoid then --If humanoid exists
		local anim = Instance.new("Animation")
		anim.AnimationId = "" --Fill this in
		local playAnim = humanoid:LoadAnimation(anim)
		playAnim:Play()
	end
end)

Local Script:

game:GetService("ServerScriptService").Script.RemoteEvent:FireServer()

#UserInputService UserInputService is much better than KeyDown (since UserInputService can get inputs from game controllers and touchscreen).

--Must be done in a local script
game:GetService("UserInputService").InputBegan:Connect(function(key)
	if key.KeyCode== Enum.KeyCode.One then
		game:GetService("ServerScriptService").Script.RemoteEvent:FireServer()
	end
end)

Use this site if you want to see the Enum index for keycodes.

Hope it helps!

#Important Edit! Apparently animations can be triggered from a local script, so remote events aren't needed. Modified code:

game:GetService("UserInputService").InputBegan:Connect(function(key)
local player = game:GetService("Players").LocalPlayer
	if key.KeyCode == Enum.KeyCode.One then
		local humanoid = player.Character:FindFirstChild("Humanoid")
		if humanoid then --If humanoid exists
			local anim = Instance.new("Animation")
			anim.AnimationId = "" --Fill this in
			local playAnim = humanoid:LoadAnimation(anim)
			playAnim:Play()
		end
	end
end)

The reason why the animation may not be running is because Animations have different priorities dictating which animation be played over another. Here is a list of enums for AnimationPriority. If you want an animation to be played, you should set its priority to something higher than the default animations (action is the highest priority). If an animation is played and another animation is played with the same priority, the first animation is stopped over the second one.

#Final Product

local anim = Instance.new("Animation")
anim.AnimationId = "" --Fill this in

game:GetService("UserInputService").InputBegan:Connect(function(key)
	local player = game:GetService("Players").LocalPlayer
	if key.UserInputType == Enum.UserInputType.Keyboard then --Make sure it is a keyboard input
   		if key.KeyCode == Enum.KeyCode.One then
   	    	local humanoid = player.Character:FindFirstChild("Humanoid")
   	    	if humanoid then --If humanoid exists
   	        	local playAnim = humanoid:LoadAnimation(anim)
   	        	playAnim.Priority = Enum.AnimationPriority.Action --Highest priority animation
   	        	playAnim:Play()
   		    end
		end
	end
end)

Change Enum...Action to Enum...Core if you want animations like walking, falling, jumping or swimming to override it.

a much better method would be doing:

local player = game.Players.LocalPlayer
repeat wait() until player.Character.Humanoid
local humanoid = player.Character.Humanoid
local mouse = player:GetMouse()
Animation = script:WaitForChild("Animation")
mouse.KeyDown:Connect(function(key)
    if key == "l" then  --REMEMBER TO INSERT A ANIMATION INTO THE SCRIPT
        local Animationtrack = game.Players.LocalPlayer.Character.Humanoid:LoadAnimation(Animation)
        Animationtrack:Play()
    end
end)