"Error: Invalid category., Details: The category selected is not a leaf category"

Within eBay, there are "tree" categories, and "leaf" categories. The "tree" category is the main category, and the "leaf" category is the sub-category. 

• For example, the "tree" category: "Books" 
• The "leaf" category: Audiobooks, Children & Young Adults, Cookbooks, Fiction & Literature...etc. 

When you are list an item, make sure that you have chosen a "leaf" category, and not a "tree" category. Once you have selected the proper category, click the blue "Save" button. Navigate back to Queued Listings, and click the blue "Submit Listings" button.