What should I do?

Here are the scripts:

Frame = ScreenGui.MainMenu

function onClicked()
	for k, p in pairs(game.Players:GetPlayers()) do
		p.PlayerGui.Frame.Visible=false
	end
end

script.Parent.MouseButton1Click:connect(onClicked)

Frame = ScreenGui.GameInfo

function onClicked()
	for k, p in pairs(game.Players:GetPlayers()) do
		p.PlayerGui.Frame.Visible=true
	end
end

script.Parent.MouseButton1Click:connect(onClicked)

Appears a message: Players.Player1.PlayerGui.ScreenGui.MainMenu.Frame.Informat:1: attempt to index global 'ScreenGui' (a nil value) and Players.Player1.PlayerGui.ScreenGui.MainMenu.Frame.Informat:1: attempt to index global 'ScreenGui' (a nil value). I don't know what will I do, is this?

game.StarterGui.ScreenGui:ChangeThingToValue(function()

Or this?

Frame.Visible:ChangeToAnotherFrameTheCurrentValue = true

Scripting is so hard for me...

this is Either HTML service or just Plain HTML :l

<pre class="brush: lua">Appears a message: Players.Player1.PlayerGui.ScreenGui.MainMenu.Frame.Informat:1: attempt to index global 'ScreenGui' (a nil value) and Players.Player1.PlayerGui.ScreenGui.MainMenu.Frame.Informat:1: attempt to index global 'ScreenGui' (a nil value). I don't know what will I do, is this?
</pre>

game.StarterGui.ScreenGui:ChangeThingToValue(function()

<pre class="brush: lua">Or this?
</pre>

I dont know much about HTML service but if there isn't something that says HTML service in there than it plain HTML and HTML is not lua which is what scripting on roblox is.