Shop Gui Won't Open, and says this when I try to open it. Why does it say this? And How do I fix it.

I am gonna start of first on why it errored, The reason it errored is because you asking the visible property of a ScreenGui (see the error), but you ScreenGui doesnt have a visible property but it does have an Enabled property who does the similar, Secondly from further inspecting your code, its most like you because you accidently forgot to wrote .Frame in you first event.

Secondly you should use Visible instead of visible since it is deprecated

(in case both are refering to the same TextLabel) Thirdly you code doenst do what you think it does atm what it does is fire both functions: it sets Visible prop to true and to false, what you probably want it do is when its visible you want it to become invisble and when its visible you want to make it invisible

script.Parent.MouseButton1Down:Connect(function() 
    script.Parent.Parent.Frame.Visible = not  script.Parent.Parent.Frame.Visible
end)

so the above sets the current bool of Visible to the oposite, so when u click it sets it to Visible to false and when u click once more it sets it to true, etc...

(in case they arent reffering to the same TextLabel)

then all you have to do is fix the first function

script.Parent.MouseButton1Down:Connect(function() 
    script.Parent.Parent.Frame.Visible = false -- i just simply added .Frame, since you forgot to add it 
end)

Maybe you could try this

script.Parent.MouseButton1Click:Connect(function(click) -- waits for click
if frame.Visible == false then -- checks if frame if not visible if it isnt visible then it will
frame.Visible = true -- make the frame visible
end
if frame.Visible == true then -- checks if the frame is visible if visible then
frame.Visible = false -- frame is visible
end
end)

so basicly if on click the script will detect if it is visible and if it is not it will make it visible vice-versa