Why are none of the commands under this if statement not running?

The GUI should be player's gui, and it is parented to the player. And your code should be inside the local script. As far as that your code is only inside this script, it would be complicated to fix your code.

--This should be the normal script, put it inside Server Script Service
local plrs = game.Players:GetPlayers()
local Event = Instance.new("RemoteEvent",workspace)
Event.Name = "GuiEvent"
--[[IF YOU WANT TO DELETE THE CAGES GLOBALLY
local deleted = false
--]]
if #plrs == 1 then --I changed it to 1 so I could test it alone.
    print"started"
    --If you want to test it to yourself, then I will only use one individual inside this statement.
    local PlrGui = plrs[1]:WaitForChild("PlayerGui")
    Event:FireClient(plrs[1])--Fire the event
	--[[IF YOU WANT TO DELETE THE CAGES GLOBALLY
	if deleted == false then
	ws.RedCage:Destroy()
    	ws.BlueCage:Destroy()
	deleted = true
	end--]]
end
--===================
--Local Script
local Event = game.Workspace:WaitForChild("GuiEvent")
local gui = game.Players.LocalPlayer:WaitForChild("PlayerGui")
--[[IF YOU WANT TO DELETE THE CAGES ON THE CLIENT SIDE
local deleted = false
--]]
Event.OnClientEvent:Connect(function()
    gui.ScreenGui.Frame.PreGame.Visible = false
    gui.ScreenGui.Frame.Timer.Visible = true
    gui.ScreenGui.Frame.BluePlayer.Visible = true
    gui.ScreenGui.Frame.RedPlayer.Visible = true
	--[[IF YOU WANT TO DELETE THE CAGES ON THE CLIENT SIDE
	if deleted == false then
	ws.RedCage:Destroy()
    	ws.BlueCage:Destroy()
	deleted = true
	end--]]
end)

Further details: Client-Server Model, Remote Functions and Events

Edit 2020/4/24 20:36 GMT+8 : Fixed typo of the code