Zone stamps are assigned to zone categories under Options -> Element Attributes -> Zone Categories.
It's likely missing because the 9 library is not loaded. Switch to a 10 stamp and you should be set.
(Also. There is a bug where zone stamps will sometimes show up missing when they were in use in the previous file you had open. In other words: A zone stamp is in use in a project. Close that project and open another. In that project, the stamp that was used in the first one is not loaded. It will show up missing, though it's not in use in the current project. This usually clears up after a save.)