Before assigning the quotes category as default at the product level you need to create the quotes category in your store. Read this article to manage quote categories.
Login to Backend
After managing the category, go to Catalogs ➤ Products, you will land on the “Products” page. To configure the existing product press the “Edit” button or else open the “Add Product” button and you will navigate to the “Products Configuration” page.
Now scroll down to the “Set Default Quote Category” field and select the category to be set as default. Only the parent clipart category can be set as default.
After configuring the default category press the “Save” button to save it in your store.