Default category is needed when you posting articles and don’t assign them to the any individual categories. So, how to set the default category?
First step is to create a category you’ll use a default, then go into the Seetings > Writing.
The next step is to go to he places where you see “The default category of posts” and select a new default category.
After this, Move the site where you are on the bottom and confirm all changes by pressing the blue button “Save changes”. Done!