This is an example of the complexity of OCL for OpenMRS users. By definition, a collection is a set of references to existing content. You can’t create a new concept or mapping directly in a collection. All new content (concept or mappings) goes into a source.
The OpenMRS Dictionary Manager tries to protect you from this confusion by automatically making a matching/paired source with your collection, so any new concepts or new mappings (including adding answers or set members), are created within your dictionary’s source before they are added to your collection.