How to make walls disappear when GUI timer runs out?

I want to make a script that when timer ends invisible walls around player disappears. Like a race

Modify this script to fit with your game.

Script (ServerScript)

--< Services
local PlayerService = game:GetService('Players')
local TweenService = game:GetService('TweenService')

--< Variables
local PartTrans = workspace.PartTrans

--<
for countdown = 20, 0, -1 do
	for index = 1, #PlayerService:GetPlayers() do
		PlayerService:GetPlayers()[index]:WaitForChild('PlayerGui'):WaitForChild('ScreenGui'):WaitForChild('TextLabel').Text = countdown
	end
	wait(1)
end
TweenService:Create(PartTrans, TweenInfo.new(1), {Transparency = 1}):Play()
PartTrans.CanCollide = false

Hope this help, to more question put bellow in commentaries.