Two possible reasons why a newly created Storefront category in storefront-catalog-m-en is not visible on the storefront site are: A) The Storefront catalog is offline - If the catalog is not active, none of its contents, including new categories, will be displayed on the storefront. B) The category does not contain available products - A category without products will typically not appear on the storefront as there is nothing to display under that category. Categories need to have at least one available product to be visible to customers.
These factors are crucial in ensuring the visibility of categories on the storefront and should be checked whenever a category does not appear as expected.