The inspector title bar (right below the buttons) indicates you are inspecting the folder—or the corkboard itself. There is no way to show keywords on the background, if that is what you are trying to do. The option you are pointing to in the menu will show keywords on the index cards. So presumably that one card on your corkboard doesn’t have any keywords assigned to it.
You evidently have been successfully adding keywords to at least one document though! You very clearly have a huge list of keywords assigned to the folder.
By the way you can drag keywords right from the Project Keywords floating panel onto index cards themselves.