Any ideas on why doesn't my GUI close itself after being opened fine?

I'm trying to get my character select GUI to disappear once a character is selected, but this script doesn't seem to work

script.Parent.MouseButton1Click:connect(function()
    local Cover = game.StarterGui.CharSelect.BlackScreen:WaitForChild('BookHolder')
    Cover.Visible = false
    game.StarterGui.CharSelect.BlackScreen.BackgroundTransparency = 1
end)

BookHolder is the frame holding the Button that is to be pressed

I've tried to set it as "Player.PlayerGui." but got no luck as it said that it couldn't find "Player" Should the code above be on a local or normal script?

this is how it is opened

script.Parent.ClickDetector.MouseClick:connect(function(Player)
    Player.PlayerGui.CharSelect.BlackScreen.BackgroundTransparency = 0
    Player.PlayerGui.CharSelect.BlackScreen.BookHolder.Visible = true
end)

this opens it fine, just don't know why it isn't closing

Thanks for help in advanced :)