What did I do wrong here?

Changing something inside the StarterGui will not change it for all of the players. Instead, you can create a function to display a message for all of the players:

function Display(msg)
	for _,v in pairs(game.Players:GetPlayers())do
		local x=v.PlayerGui:FindFirstChild("Ticker")
		if x then
			x=x:FindFirstChild("Ticker")
			if x then
				x=x:FindFirstChild("TextLabel")
				if x then
					x.Text=msg
				end
			end
		end
	end
end
function Flood()
	game.Workspace.Flood.Size = Vector3.new(518, 22, 521)
	game.Workspace.Flood.CFrame = CFrame.new(Vector3.new(-1,10,-2.5))
end
function AntiFlood()
	game.Workspace.Flood.Size = Vector3.new(10, 5, 10)
	game.Workspace.Flood.CFrame = CFrame.new(Vector3.new(-1,-42,-2.5))
	Display("You lived!")
end
while true do
	wait(6)
	Display("Disaster: Flood")
	wait(9)
	Display("You have 60 seconds to build")
	wait(9)
	wait(60)
	Flood()
	wait(20)
	AntiFlood()
end