When trying to add cross-model relations to a shared model package or project, the option Add Cross-model Relation is not available when right-clicking an object. Nor can cross-model relations be added using the Cross-model Relations cross reference table. Example:
If the option for creating cross-model relations is not available in the context menu of an object, or if you cannot add the cross-model relations using the cross-reference table, it means that you do not have sufficient edit permissions in your model package or project to add them. When cross-model relations are added, they are registered in the model package or project directly under the model package level, meaning that you also need edit permissions on model package level.
If you, for example, only have edit permissions for the model in which you want to add the cross-model relations, it will not be sufficient. This is illustrated in the figure below. The user only has edit permissions for model 'ArchiSurance', the other sections of the shared model are colored gray, meaning no edit permissions, including the cross-model relations.
In order to be able to add cross-model relations to a shared model package or project, you need edit permissions on model package level. Ask the person who can manage your access rights for the model package or project to grant you the permissions. The figure below shows an example of sufficient rights for adding cross-model relations. The user has edit permissions on model package level, meaning that the user has edit permissions for everything below it, including cross-model relations.