In the given UML package diagram fragment, various elements are listed with stereotypes indicating their nature («class», «interface»). Stereotypes are used in UML to extend the vocabulary of UML in order to create new kinds of building blocks. They are typically enclosed in guillemets (« »).

The last element, 'six', does not have a stereotype indicating whether it's a class, interface, or some other kind of element. This lack of a stereotype leads to ambiguity about the nature of 'six'. Therefore, the fragment is considered invalid due to this omission.

All the other options do not constitute an error in the UML fragment: A) A list of elements is allowed in a package body. B) Visibility is not required for every element, especially within package diagrams. C) Interfaces are allowed in packages. D) Private elements are allowed in a package. E) Protected elements are also allowed in a package.

Therefore, the correct answer is:

F. Element six is missing its stereotype.