If you want to add a main category follow these steps:
1. Click first on ‘Catalog’ and then on ‘Categories’.
2. On the top left, you can click the ‘Add Root Category’ button.
3. You can then enter the category name in the 'Category Name' section and the rest of the category information in the sections below (such as 'Content', 'Display Settings', and 'Products in Category').
Note: when you are done do not forget to click on the 'Save' button to save your data.