If you want the checkerboard enable the transparency grid in the comp panel. It's the icon at the bottom that looks like a checkerboard. You could also just put another layer below the layer with the mask.
I see a bunch of keyframes on what I think is your masked layer so I think you are using the Mask Tracker. Cropped screenshots don't tell us much. The best screenshots include selecting the problematic layer, pressing 'u' twice to show us all the modified properties of the layer, making sure that the switches and modes columns are visible in the timeline (the first two icons at the bottom left corner of the timeline) and if there is any parenting going on the Parent/Link column should also be visible (Shift + F4). If just turning on the transparency grid or dropping another layer below the masked layer does not prove that you have transparency then post a full UI screenshot.
By the way, even though that tutorial has about 150K views, I'd give it a C-. It's not very good, it's poorly explained, and the workflow is not very efficient.