thank you for your answer. However, the delete option does not appear (it is greyed out). I waited 30 minutes(!), because I realized that sometimes it needs a few seconds to become click-able. Did not happen. There were no collection objects associated. I am also the admin of this database so I should have all the rights. No one else was logged in at that time.
The delete option does work when I try to delete a genus or species. I think, the bigger the tree is the longer the delete option needs to appear. If you try to delete a whole kingdom, division, or phylum it takes forever.