Oracle Database 12.2 -- RMAN-06820 with Connect identifier not configured

Problem

While taking backups from a standby database, the following error reported:

Connect identifier for DB_UNIQUE_NAME <primary_db_unique_name> not configured
RMAN-06820: warning: failed to archive current log at primary database

Solution

Set the connect identifier for the primary database from the tnsnames.ora file at the standby site:

set serveroutput on
declare n number;
begin
  n := dbms_backup_restore.setconfig(
     'DB_UNIQUE_NAME',
     '"<primary_db_unique_name>" CONNECT IDENTIFIER "<primary_tns_alias>"');
  dbms_output.put_line(n);
end;

Verify with:

select * from v$rman_configuration;

Additional info:

Oracle Database 12.2 -- RMAN-06820 When Taking Backups at a Standby Database