attempt to index nil with 'GetMouse' ?

I was trying to make an animation for my physics game. I was making a crouch animation. Can someone take a look at the script if possible.

https://pastebin.com/aAyRgVpu

The problem is that the script loads faster than the player, so you're trying to get the player's mouse when the player doesn't exist. However, you should use UserInputService if you want to get a key press event. There's also no reason to create the animation each time the key is pressed, instead, create it once and have it play or stop each time the key is pressed.

LocalScript:

local plr = game:GetService('Players').LocalPlayer
repeat wait() until plr and plr.Character and plr.Character:FindFirstChild('Humanoid')
local hum = plr.Character.Humanoid

local UIS = game:GetService('UserInputService')

local Animation = Instance.new('Animation')
Animation.AnimationId = 'rbxassetid://123'
local AnimTrack = Humanoid:LoadAnimation(Animation)

UIS.InputBegan:Connect(function(input,gp)
	if input.KeyCode == Enum.KeyCode.C then
		AnimTrack:Play()
	end
end)
UIS.InputEnded:Connect(function(input,gp)
	if input.KeyCode == Enum.KeyCode.C then
		AnimTrack:Stop()
	end
end)

You may change

mouse.KeyDown:Connect(function(Key) 

to

MouseButton1Click:Connect(function(Key) --Change the 1 to 2 if you want it to right click