If you ever see a message where it says:
"There are currently no products to display within this category!"
This usually means that you have checked an option under your categories tab that is making the sub-categories not appear.
To check, go to your admin and go to Inventory > Categories
Using the image above as an example:
If you have a PARENT category (in our case, Ixalan Block) that has the option, "Category may contain products" checked, then your sub-categories will appear hidden when you click on Ixalan Block.
To make those SUB-categories appear (Ixalan and Rivals of Ixalan), you will need to uncheck the "Category may contain products" option.
N.b - If the admin does not let you uncheck this option, this means that you do have products in this PARENT category. Those products will need to be moved out before you can uncheck this option.
For more information, please click the link for finding products that are within my parent category.