The GUI doesn't open, even with a ClickDetector and function. Why?

BACKSTORY: I am trying to make a service that is like CheckMeIn (A service that checks you in to a hotel room). I was working on the gui, so I made a script. The GUI is supposed to be a Windows styled GUI system. The script is shown below:

gui = game.StarterGui.PCTabGui
clickable = script.Parent
gui.Enabled = false;

function onMouseClick()
gui.Enabled = true;
end

function close()
	gui.Enabled = false;
end


clickable.ClickDetector.MouseClick:connect(onMouseClick)
gui.Frame.Minimize.MouseEnter:connect(close)

This appears a few times in the output in red: cloud_1685949788.ToDoListPlugin.NexusPluginFramework.NexusProject:253: Requested module experienced an error while loading

The GUI also doesn't show up.

Could anybody help me in this?

Your issue is the very first line. You get the UI from StarterGui which is where the UI gets copied from, not the actual UI that is being displayed to the user at the moment. In other words, when the player joins the game, all the contents from StarterGui gets copied to PlayerGui which is where the actual UI gets displayed. To alter the player UI you would need to access this folder which is accessible from game.Players.LocalPlayer.PlayerGui using a LocalScript.

as i see here, you define GUI as the StarterGui Instead of playergui and also, Click detector only works on script and you cant use local script for it. And i think the error on the output come from a plugin. I suggest you to find a tutorial on youtube