Attempted to index nil with playergui?

@virushunter9 is correct! To get the people who fall into the pit, you can get the part that touched cc. Then, check if the parent of it has a humanoid and it is a real player.

local Players = game:GetService("Players")
local SoundService = game:GetService("SoundService")

local c = workspace.htisguy.Sound
local o
local cc = workspace.htisguy
local k = SoundService.Sound

cc.Touched:Connect(function(limb)
	local character = limb.Parent
	local humanoid = character:FIndFirstChildWhichIsA("Humanoid")
	local player = Players:GetPlayerFromCharacter(character)

	if character and humanoid and player then -- if it's a real player
		o = player
		
		k:Pause()
		c:Play()
		task.wait(0.5)
		o.PlayerGui.warp.Enabled = true
	end
end)

But if you want to play the sound ONLY to the player who touched it, you can make it a local script and put it in StarterGui or StarterPlayerScripts because local scripts won't work in workspace.