SQL> select log_mode,open_mode , database_role from v$database;
LOG_MODE OPEN_MODE DATABASE_ROLE
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ARCHIVELOG READ ONLY WITH APPLY PHYSICAL STANDBY
If you got the above output then your database is physical standby which is READ ONLY mode. You can only select when the database is in Read only mode. If you want to do any changes then you need to change mode to READ WRITE and recreate your standby database. Else do the changes on primary database which will reflect the changes on Standby database.