Error: ORA-16831: operation not allowed on this member

When trying to add a standby database to your configuration, I received

DGMGRL>  add database 'cdb_stby1' as connect identifier is 'cdb_stby1';
Error: ORA-16831: operation not allowed on this member

This message means that the database you are trying to add, in this case cdb_stby1, is not ready to become a standby database in your configuration.

It is easy to verify by querying the "database_role" column in the v$database view.

It needs to be opened with a role that is NOT primary, for example, the PHYSICAL STANDBY role

select database_role, open_mode,name from v$database;

DATABASE_ROLE    OPEN_MODE            NAME
---------------- -------------------- ------------------------------
PHYSICAL STANDBY MOUNTED              CDB

If your database is opened with the role PRIMARY, you need to clone it from your target, for example by using RMANs DUPLICATE ... FOR STANDBY FROM ACTIVE DATABASE