ORA-20000

Use function dbms_isched.get_credential_password:

SQL> 
SQL> begin
  2    dbms_scheduler.create_credential(credential_name => 'credential4test',
  3                                     username        => 'user4test',
  4                                     password        => 'pass4test');
  5  end;
  6  /
 
PL/SQL procedure successfully completed
 
SQL> 
SQL> select user$.name as credential_owner,
  2         obj$.name  as credential_name,
  3         scheduler$_credential.username,
  4         scheduler$_credential.password,
  5         dbms_isched.get_credential_password(obj$.name, user$.name) enc_password
  6  from   scheduler$_credential,
  7         obj$,
  8         user$
  9  where  user$.user# = obj$.owner# and
 10         scheduler$_credential.obj# = obj$.obj#;
 
CREDENTIAL_OWNER   CREDENTIAL_NAME  USERNAME     PASSWORD                                ENC_PASSWORD
------------------ ---------------- ------------ --------------------------------------- --------------
SYS                CREDENTIAL4TEST  user4test    BUG19SWE76fBKqdqgKsZu/fTpM8wOfSQXg==    pass4test

SQL>