This is a warning that reports that the category you have selected is invalid, and it usuall means you haven't selected a leaf category.  To list on eBay you can't list in a main category, or one that contains sub-categories.  It has to be a section that has no sub-categories.

In Flexible Lister you will usually see a combination of these errors:

WARNING:  - [Warning] - 20169: Invalid shop category ID, 0, since it is non-leaf.  So item has been listed to the 'Other' store category
WARNING:  - [Warning] - 2191507: Category ItemSpecifics does not support Item Specifics.  Input values have been ignored
WARNING:  - [Error] - 87: The category selected is not a leaf category.

When you get this, you need to is review the categories you are listing in.