I have a problem of getting the player's playergui?

I'm making a shackle tool and I need to access the prisoner's playerGui when I click on him with the shackles. I'll mark the place where I'm having troubles. Could you help me figure out what's wrong?

local shackles = script.Parent
local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()
local canJail = false
local studsToArrest = 20

function onEquipped()
	print("Shackles equipped")
	canJail = true
end

function onUnequipped()
	print("Shackles unequipped")
	canJail = false
end

function onActivation()
    if mouse.Target.Parent:FindFirstChild("Humanoid") then
		local prisoner = mouse.Target.Parent
		local mag = (prisoner.Torso.Position - plr.Character.Torso.Position).magnitude
	if mag <= studsToArrest then
		if prisoner:FindFirstChild("timesJailed") then
			prisoner.timesJailed.Value = prisoner.timesJailed.Value + 1
		else
			local tj = Instance.new("NumberValue", prisoner)
			tj.Name = "timesJailed"
			tj.Value = 1
		end
		
		if game.Workspace.Jail.jailSpawn1.Occupied.Value == false then
			prisoner.Torso.CFrame = game.Workspace.Jail.jailSpawn1.CFrame + Vector3.new(0,2,0)
			game.Workspace.Jail.jailSpawn1.Occupied.Value = true
		elseif game.Workspace.Jail.jailSpawn2.Occupied.Value == false then
			prisoner.Torso.CFrame = game.Workspace.Jail.jailSpawn2.CFrame + Vector3.new(0,2,0)
			game.Workspace.Jail.jailSpawn2.Occupied.Value = true
		elseif game.Workspace.Jail.jailSpawn3.Occupied.Value == false then
			prisoner.Torso.CFrame = game.Workspace.Jail.jailSpawn3.CFrame + Vector3.new(0,2,0)
			game.Workspace.Jail.jailSpawn3.Occupied.Value = true
		elseif game.Workspace.Jail.jailSpawn4.Occupied.Value == false then
			prisoner.Torso.CFrame = game.Workspace.Jail.jailSpawn4.CFrame + Vector3.new(0,2,0)
			game.Workspace.Jail.jailSpawn4.Occupied.Value = true
		elseif game.Workspace.Jail.jailSpawn5.Occupied.Value == false then
			prisoner.Torso.CFrame = game.Workspace.Jail.jailSpawn5.CFrame + Vector3.new(0,2,0)
			game.Workspace.Jail.jailSpawn5.Occupied.Value = true
		elseif game.Workspace.Jail.jailSpawn6.Occupied.Value == false then
			prisoner.Torso.CFrame = game.Workspace.Jail.jailSpawn6.CFrame + Vector3.new(0,2,0)
			game.Workspace.Jail.jailSpawn6.Occupied.Value = true
		else
			print("All prison cells are full")
			if plr.PlayerGui:FindFirstChild("Notifier") then
				if plr.PlayerGui.Notifier == false then
					plr.PlayerGui.Notifier.Enabled = true
					for i = 1,0,-0.1 do
						plr.PlayerGui.Notifier.notifier.TextTransparency = i
						wait()
					end
					wait(2)
					for i = 0,1,0.1 do
						plr.PlayerGui.Notifier.notifier.TextTransparency = i
						wait()
					end
				end
			end
		end
		
		print(prisoner.Name.." has been jailed "..prisoner.timesJailed.Value.." times now...")
		game.Players:GetPlayerFromCharacter(prisoner).TeamColor = BrickColor.new("Really black")
		
		if prisoner.timesJailed.Value == 1 then
			local prisonerPlr = game.Players:GetPlayerFromCharacter(prisoner)
			for i,v in pairs(prisonerPlr:GetChildren()) do
				print(v.Name) --I print out the children here
				wait()
			end
			prisoner.PlayerGui.JailTimer.Frame.Visible = true --The error says playerGui doesnt exist in the player.
			for i = 120,0,-1 do
				print(i)
				prisonerPlr.PlayerGui.JailTimer.Frame.Timer.Text = i
				wait(1)
			end
		end
	end
	end
end

shackles.Equipped:connect(onEquipped)
shackles.Unequipped:connect(onUnequipped)
shackles.Activated:connect(onActivation)

If I am correct it seems that you are trying to find inside of a player's CHARACTER

you see... Guis are contained in the player, not character

prisonerPlr.PlayerGui.JailTimer.Frame.Visible = true

see how i made it prisonerPlr?

It was a simple mistake really....

To explain the error, you weren't accessing the PlayerGui from the 'prisonerPlr' variable.

But unfortunately if FilteringEnabled is enabled as it should be, you cannot directly manipulate anything on the client. You must use remotes.

• Setup a RemoteEvent in ReplicatedStorage.

• Use FireClient from the server.

• Change the UI's visibility in an OnClientEvent event from the client.

Server:

local remote = game.ReplicatedStorage.TimerRemote --This is your RemoteEvent

--codecodecode

if prisoner.timesJailed.Value == 1 then
	local prisonerPlr = game.Players:GetPlayerFromCharacter(prisoner)
	--print children
	if prisonerPlr then --Make sure they exist
		remote:FireClient(prisonerPlr) --FireClient
	end
end          

Client:

######'frame' should be equivalent to what was prisonerPlr.PlayerGui.JailTimer.Frame

local remote = game.ReplicatedStorage.TimerRemote --This is your RemoteEvent
local frame = script.Parent
local timer = frame:WaitForChild("Timer")

remote.OnClientEvent:Connect(function() --OnClientEvent
	frame.Visible = true
	for i = 120,0,-1 do
		print(i)
		timer.Text = i
	end
	frame.Visible = false
end)