The relationship between the sde and user-schema geodatabases in Oracle

You can no longer create or upgrade user-schema geodatabases.

User-schema geodatabases are those stored in the schema of a user other than the sde user in Oracle. They are not entirely self-contained; there is some functionality, such as user-defined types, that is only in the sde geodatabase. For that reason, you cannot treat geodatabases in users' schemas as their own entity, and you cannot upgrade the sde geodatabase until you move content out of a user-schema geodatabase and delete the user-schema geodatabase from the Oracle database.

See Move user-schema geodatabases to stand-alone geodatabases in Oracle for more information.