add Subcategory to Category

To add Subcategories to Categories, go to Admin Dashboard / Catalog / Categories.  Then use the "+ Add New" button to add all the categories and subcategories You want. All of them will be shown as Categories in the first column (Name). For the Category You want to convert into a Subcategory, click in the third column (Parent) on the drop-down menu and select the Category You want to add as the parent one (the one which will be the main). 

For example, if You want to add the "Bath" category as a subcategory of the "Gift" category, in the "Bath" row under the name "Parent" click on the drop-down menu and select the "Gift" category below which You want to add it. This will be automatically saved.  When You set all the categories and subcategories as You want, in the upper right corner, click the "Rebuild" button, it will save all changes and clear the cache.