An << extend >> UseCaseDependency must reference at least one ExtensionPoint.
An ExtensionPoint is a feature of a UseCase that identifies a point where the behaviour of a UseCase can be augmented with elements of another (extending) UseCase. Therefore, an << extend >> UseCaseDependency should reference an ExtensionPoint of the UseCase it extends.
Add an ExtensionPoint to the UseCase you want to extend if none already exist, and/or reference that ExtensionPoint in the UseCaseDependency properties.