Message Oracle LSX-00157 - ~S is abstract and cannot be used in an instance document

ERROR~S is abstract and cannot be used in an instance document
CAUSEThe element declaration or the type definition was abstract.
ACTIONWhen an element is declared to be abstract, a member of that elements substitution group must appear in the instance document. When an elements corresponding type definition is declared as abstract, all instances of that element must use xsi: type to indicate a derived type that is not abstract.