You can not create a sub-category once you associate or add products to the main category.

To associate products with the main category

  1. Go to the Catalog page.
  2. Click the desired category name page to open the category details page.
  3. Select the category from the list.
  4. From the right panel, click the Products.
  5.  Click Add Price List to open the Add Products to Category popup window.
  6. Select the required price lists by clicking the check boxes next to the products. 
  7. Click Add.