When importing a game or product line you don't already have a product type or category for, you will want to follow these steps which we'll go through in detail below:
1. Create the product type on your Inventory > Types page
2. Create the parent category, assigning to it the product type you created in step 1.
3. Import the set, using the "create a new category" option to create a subcategory under the parent category created in step 2.
On your Inventory > Types page, you'll want to check and see if you have a product type for the game (we'll use Argent Saga TCG in this example). As a rule of thumb, the product type should match the parent category on the Import page. In this case, the parent category importing from is Argent Saga Singles.
On the Inventory > Types page, you would click on "New Product Type", enter Argent Saga Singles as the name, skip the descriptors for now, and set up your Conditions. All product types have a "New" condition value by default, which you can rename. Then you can click on the "New Condition" button to add additional values. Finally, click "Save Product Type".
On the Inventory > Categories page, click the "New Category" button. For the parent category, leave it as "create as top level category". Name it (probably the same name as the product type), and select the Argent Saga Singles product type just created. You can uncheck the "hide from buylist box" if you wish, but you must keep the "category may contain products" box unchecked. Click "Create Category" when finished.
Note: it's important that you only create the parent category. Don't create any subcategories underneath it, or the import will give you the "missing descriptors" error.
Finally, head to your Inventory > Import page. Select the Argent Saga Singles category, and then the subcategory for the set. Click "Apply Search" in step 2, and then "Add All" once the products appear. In the final step of import, select the option for "Create a new category". Select your newly created Argent Saga Singles category, and enter the set name into the Category Name field. Then click "Import".