Am an amateur here. Have been trying to pull out an image into a transparent background. I tried control J. Then I tried to add mask to 'reveal selection' but it would not permit me to open that option.
For duplicating a layer (crtl-j) and for creating a new layer mask, if you have a selection active, it should try to take only that selection... so if you had a selection and duplicated the layer, you would end up with a new layer with only the selected pixels copied to it. Did you duplicate the whole layer without a selection first? When you try to add a mask, do you have the parts you want to show selected (blinking-selection-outline on the image)? You still have to turn off the layer below afterwards to get transparency.