Trying to make timer, but doesn't update text, and no errors in the output, how can I fix this?

I'm trying to make a timer, when a player buys something, but it doesn't work and it gives no errors.

There are two scripts one local, other is server.

Here is my local script (where a remote function gets fired):

-- Varabiles
local plr = game.Players.LocalPlayer

local replicatedStorage = game:GetService('ReplicatedStorage')
local remoteFunction = replicatedStorage:WaitForChild('PotionRemotes'):WaitForChild('DoubleClicks'):WaitForChild('TimerRemote')


game.Players.PlayerAdded:Connect(function(plr)
	-- keeps firing the remote function.
	while plr.Name do
		wait(1)
		remoteFunction:InvokeServer(plr)
	end
end)

Here is the server script:

-- replicated storage varaibes
local replicatedStorage = game:GetService('ReplicatedStorage')
local remoteFunction = replicatedStorage:WaitForChild('PotionRemotes'):WaitForChild('DoubleClicks'):WaitForChild('TimerRemote')



remoteFunction.OnServerInvoke = function(plr)
	-- time varabiles
	local timeLeft = game.ServerStorage.DataFolder:WaitForChild(plr.Name):WaitForChild('PotionsFolder'):WaitForChild('DoubleClicksTimeLeft')
	local isActive = game.ServerStorage.DataFolder:WaitForChild(plr.Name).PotionsFolder:WaitForChild("DoubleClicksActive").Value
	local timeLabel = plr.PlayerGui.DisplayGUI.DoubleClicksTimerFrame.TimeLabel
	local timerFrame = plr.PlayerGui.DisplayGUI.DoubleClicksTimerFrame
	-- checks if a bool value is true
	if isActive.Value == true then
		-- sets some guis active, and visible
		timerFrame.Active = true
		timeLabel.Active = true
		timerFrame.Visible = true
		
		-- checks if there is anymore time left
		if timeLeft.Value >= 1 then
			-- decreases
			timeLeft.Value = timeLeft.Value - 1
			-- updates text
			timeLabel.Text = timeLeft.Value..' sec left'
		else
			-- if not it'll set is active to false, and the frame invisible.
			isActive.Value = false
			timerFrame.Visible = false
		end
	end
end

Also the timer decreasing and updating text has to be in a server script so I can access the int values, and bool valaues.

All UI changes (with the exception of BillboardGuis and SurfaceGuis) must be made on the client. After deciding what to put on the UI you should send a remote event from the server to the client to update that UI. If you want the change to be made for all player's UI, just use :FireAllClients().

I don't think changing gui on a server script would be very good. I think you can use FireAllClients() instead.