Gradient tool does nothing inside of reveal all layer mask

Hello, I am trying to blend two images together.

I add the images. I create a "reveal all" layer mask on the top one. I click inside the layer mask box. I select my gradient (black to white) tool and drag it across the image. Nothing happens to the image or inside the layer. 

When YouTubers instruct me how to do this, their images blend and their layer mask boxes becomes half black and half white. Mine is still full white, the gradient tool has no effect. 

I have tried resetting gradient tool and selecting different things. Don't know how to use photoshop, just trying to go above and beyond for a work assignment. 

Thanks!