The error message in question is:
"This is a top level category. We're sorry, but eBay does not permit detailed research on this category. Please select another category or use the Search report to research listings in this category."
As the message describes, Terapeak is prevented from providing category research for top level categories by the marketplace.
Since the category in question does not contain any lower level branches, research in that part of eBay's market is restricted.
Listing data related to the category in question is still available to research through Product and Hot Research reports.