Animation after pressing a key doesn't work?

There are multiple error in your code.

First of all, you're getting OnServerEvent on ReplicatedStorage, not a RemoteEvent. In my revised working code below, I created a Remote Event in ReplicatedStorage called "punchingEvent".

Also, you were trying to get "KeyDown" on a Mouse, not a keyboard. You should use UserInputService for this, and use "InputBegan". When the input begins, you can check if it was a key, and what key they pressed. Then, the "q" should be Enum.KeyCode.Q because the KeyCode is an Enum.

Lastly, AnimationID isn't a valid member of animation, it's AnimationId. It's case sensitive.

FYI: You can get character from the player object, you don't have to find it in workspace.

That error you received was from a plugin you had installed, not your code, unless your code is a plugin.

It gave me an error because I don't own the animation, but I hope you do so you can run it!

Here's the revised code:

Animation (ServerScriptService) Script

game.ReplicatedStorage.punchingEvent.OnServerEvent:Connect(function(player, animationID)
    local animation = Instance.new("Animation")
    animation.AnimationId = "http://www.roblox.com/Asset?ID"..animationID

    local loadedAnimation = player.Character.Humanoid:LoadAnimation(animation)
    loadedAnimation:Play()
end)

Controls (StarterGui) Local Script

local UIS = game:GetService'UserInputService'

UIS.InputBegan:connect(function(input)
	if input.UserInputType == Enum.UserInputType.Keyboard then
		local keyPressed = input.KeyCode
    	if keyPressed == Enum.KeyCode.Q then
        	game.ReplicatedStorage.punchingEvent:FireServer(01900686745)
		end
    end
end)

I hope this helped!