Why Won't C to Crouch Work in Server, But it Does in Client?

Try adding a fireserver function.

Like this :

	local UserInputService = game:GetService("UserInputService")
	local Player = game.Players.LocalPlayer
	local Character = Player.Character or Player.CharacterAdded:wait()
	local Humanoid = Character:WaitForChild("Humanoid")
	local CrouchStart = game.ReplicatedStorage.("your remote event")
	local CrouchEnd = game.ReplicatedStorage.("your other remote event")	


	 
	local function CheckKeyDown(Key)
	    if Key.KeyCode == Enum.KeyCode.C then
		  local Animation = script:WaitForChild("Animation")
		  CrouchStart:FireServer(Animation)
	    end
	end
	 
	local function CheckKeyUp(Key)
	    if Key.KeyCode == Enum.KeyCode.C then
	      	  local Animation = script:WaitForChild("Animation")
		  CrouchEnd:FireServer(Animation)
	    end
	end
	 
	UserInputService.InputBegan:Connect(CheckKeyDown)
	UserInputService.InputEnded:Connect(CheckKeyUp)

Then add a script in the serverscriptstorage :

local CrouchStart = game.ReplicatedStorage.("your remote event")
local CrouchEnd = game.ReplicatedStorage.("your other remote event")

CrouchStart.OnServerEvent:Connect(function(plr, Animation)
	local char = plr.Character or plr.CharacterAdded:Wait()
	local hum = char:WaitForChild("Humanoid")
	local Track = hum:LoadAnimation(Animation)
	Track.Priority = Enum.AnimationPriority.Action
	Track.Looped = true
	Track:Play()
	hum.WalkSpeed =8
end)

CrouchEnd.OnServerEvent:Connect(function(plr, Animation)
	local char = plr.Character or plr.CharacterAdded:Wait()
	local hum = char:WaitForChild("Humanoid")
	local Track = hum:LoadAnimation(Animation)
	
	Track.Looped = false
	Track:Stop()
	hum.WalkSpeed = 16
end)

Comment if there is an error :)