Mouse1OnClick doesn't work for me when I clicked on the button. I did very simply, but it did not work. My script was stored in, StarterGUI --> ScreenGUI ---> Frame --> Button --> Script. The print did not output in the Output Page.
script.Parent.MouseButton1Click(function() print("Test") end)
You did not use a Connect
script.Parent.MouseButton1Click:Connect(function() print("Test") end)