How do I make my click detector script work for gui enabling?

Errors: You can not access the PlayerGui because Gui's are set by the client. Maybe some Gui's created by the server you can access but most of the Gui's that we use is created with client.

You need to use RemoteEvents with FireClient

You can see RemoteEvents and FireClient on roblox wiki:

RemoteEvents/RemoteFunctions

FireClient

For this, you need to create a Remote Event, LocalScirpt, ClickDetector & Script

For first create a RemoteEvent and put it on Replicated Storage, now create a LocalScript and put it on StarterGui, on LocalScript located in StarterGui put this code:

-- FOR USE THIS PUT IN StarterGui --

repeat wait() until game.Players.LocalPlayer -- Not return nil for player

local event = game:GetService("ReplicatedStorage"):WaitForChild("Event") -- Put your event here

local plr = game.Players.LocalPlayer -- Get player

event.OnClientEvent:Connect(function() -- On fire the client

    local gui = "BookGui" -- Put your GUI Name here

    plr.PlayerGui:WaitForChild(tostring(gui)) -- Wait for GUI

    if plr["PlayerGui"]:FindFirstChild(tostring(gui)) then -- Find gui
        local gui = plr["PlayerGui"][tostring(gui)]
        gui.ImageLabel.Visible = true -- set ImageLabel visibility to true
        gui.TextButton.Visible = true -- Set TextButton visibility to true
    end
end)

Or you can put it on GUI and put this code:

-- FOR USE THIS PUT IN YOUR GUI --

wait()
local event = game:GetService("ReplicatedStorage"):WaitForChild("Event") -- Put your event here

event.OnClientEvent:Connect(function() -- On fire the client
    script.Parent.ImageLabel.Visible = true -- set ImageLabel visibility to true
    script.Parent.TextButton.Visible = true -- Set TextButton visibility to true
end)

Now create a Script and ClickDetector In your part and put this:

local event = game:GetService("ReplicatedStorage"):WaitForChild("Event") -- Put your event here

local clickdetector = script.Parent.ClickDetector -- Click detector location

clickdetector.MouseClick:Connect(function(plr)
    event:FireClient(plr)
end)

Hope it helped!

You got that error, cuz you can't access to player's stuff from script(server script). If you want to do that, you need to use remote events. It is very easy.

For first insert RemoteEvent in ReplicatedStorage and change it name what you want. Now type this into ur clickdetector script

local Event = game:GetService("ReplicatedStorage"):WaitForChild("Your Event Here") --your remote event name here

function Clicked(player)
    Event:FireClient(plr) --giving exept player info that he clicked on the part
end

script.Parent.MouseClick:Connect(Clicked)

Now insert a localscript into startergui and type in

local Event = game:GetService("ReplicatedStorage"):WaitForChild("Your Event Here") --your remote event name here
local player = game.Players.LocalPlayer

Event.OnClientEvent:Connect(function() --this script starts when he get info from server
    player.PlayerGui:WaitForChild("BookGui").ImageLabel.Visible =true
    player.PlayerGui.BookGui.TextButton.Visible =true
end)