The category selected is not a leaf category" and "The category is not valid, select another category

The errors may arise after eBay makes category changes. To comply with eBay catalog, please timely update the marketplace data in your M2E Pro: under eBay > Configuration > Marketplaces, please click Update Now.

If the update didn't work, make sure you selected a leaf-level category, i.e. the category that has no subcategories.