How to enable/disable database Audits?

What exactly do you want to audit?
If you mean DB size Audit, then SELECT audit_license_size();
Will trigger an immediate audit of the database size and
select GET_COMPLIANCE_STATUS();
Will display whether your database size follows your Vertica license agreement based on the last audit.
See: https://www.vertica.com/docs/10.0.x/HTML/Content/Authoring/AdministratorsGuide/Licensing/CalculatingTheDatabaseSize.htm

If you mean Audit the requests history, do SELECT DATA_COLLECTOR_HELP(); to get online usage instructions about the Data Collector, the DATA_COLLECTOR system table, and the Data Collector control functions.
For example, you can get a history of all SQL requests issued from a data_collector table name: dc_requests_issued
And a history of all completed requests from table: dc_requests_completed
To see the relevant tables component do:

SELECT DISTINCT table_name, component, description FROM data_collector where table_name ilike '%requests%';
        table_name         |      component       |                     description
---------------------------+----------------------+------------------------------------------------------
 dc_lock_requests          | LockRequests         | History of lock requests
 dc_requests_retried       | RequestsRetried      | History of all SQL requests issued that were retried
 dc_mergeout_requests      | MergeoutRequests     | History of mergeout request tracker requests
 dc_requests_issued        | RequestsIssued       | History of all SQL requests issued
 dc_import_export_requests | ImportExportRequests | History of import/export requests
 dc_requests_completed     | RequestsCompleted    | History of all SQL requests completed
(6 rows)

To see those components collector retention policy do:

    SELECT get_data_collector_policy('requestsissued');
                              get_data_collector_policy
    ------------------------------------------------------------------------------
     2000KB kept in memory, 256000KB kept on disk. Time based retention disabled.
    (1 row)
SELECT get_data_collector_policy('RequestsCompleted');
                          get_data_collector_policy
------------------------------------------------------------------------------
 2000KB kept in memory, 256000KB kept on disk. Time based retention disabled.

Data Collector retention policies hold the following information:
1. Which component to monitor
2. How much memory to retain
3. How much disk space to retain
Explained here: https://www.vertica.com/docs/10.0.x/HTML/Content/Authoring/AdministratorsGuide/Monitoring/Vertica/ConfiguringDataRetentionPolicies.htm

Data Collector is on by default and retains information for all sessions.
A superuser can disable Data Collector at any time.
To disable the Data Collector: ALTER DATABASE DEFAULT SET EnableDataCollector = 0;
To re-enable the Data Collector: ALTER DATABASE DEFAULT SET EnableDataCollector = 1;

@mosheg, Above information is so helpful.
In the data_collector/interval_time, if the value is greater than 0 then is it days/hours?

I am looking for the below audits options:
1. Failed login attempts and the reason.
2. Insert, Update, Delete SQL's
3. Truncate & Drop
4. Login/logoff
5. Failed attempt on the above #2 & #3

1. Consider the following example:

   CREATE USER Mike IDENTIFIED BY 'MikePasword';
   CREATE USER
   vsql -U Mike -w BadPas
   Will get an error message: vsql: FATAL 3781: Invalid username or password
   This will be reflected in the last vertica.log using for example the following command:
   grep -B 3 -e "Invalid user" $(admintools -t list_db -d EEVDB | grep -e "Catalog Dir" | cut -d: -f2)/vertica.log
   2020-05-15 18:36:45.941 Init Session:0x7ef8fb117700 @v_eevdb_node0001: 00000/4686: Authentication - sendAuthRequest: user=Mike database=dbadmin host=::1 authType=3
   2020-05-15 18:36:45.942 Init Session:0x7ef8fb117700-a0000000001984 [Txn] Begin Txn: a0000000001984 'check_login_history'
   2020-05-15 18:36:45.942 Init Session:0x7ef8fb117700-a0000000001984 [Txn] Rollback Txn: a0000000001984 'check_login_history'
   2020-05-15 18:36:45.942 Init Session:0x7ef8fb117700 @v_eevdb_node0001: {SessionRun} 28000/3781: Invalid username or password

   In addition to the record in table: dc_login_failures

   vsql -xc "select * from dc_login_failures;"
   -[ RECORD 1 ]--------------+------------------------------
   time | 2020-05-15 18:36:45.942189-04
   node_name | v_eevdb_node0001
   database_name | dbadmin
   user_name | Mike
   client_hostname | ::1
   client_pid | 479837
   client_label |
   client_version |
   client_os_user_name | dbadmin
   ssl_client_subject |
   ssl_client_fingerprint |
   authentication_method | Password
   client_authentication_name | default: Password
   requested_protocol | 3.8
   effective_protocol | 3.8
   reason | FAILED

2. Consider the following example:

   SELECT * FROM dc_requests_issued where left(request,10) ilike '%insert%' and (NOW() - time) < '24 hours';
   -[ RECORD 1 ]-----+----------------------------------------------------------
   time | 2020-05-15 19:10:48.472227-04
   node_name | v_eevdb_node0001
   session_id | v_eevdb_node0001-290527:0x163ae
   user_id | 45035996273704962
   user_name | dbadmin
   transaction_id | 45035996273711574
   statement_id | 1
   request_id | 3
   request_type | QUERY
   label |
   client_label |
   search_path | "$user", public, v_catalog, v_monitor, v_internal, v_func
   query_start_epoch | 62
   request | insert into my_test values(1);
   is_retry | f

3. SELECT * FROM dc_requests_issued where UPPER(left(request,8)) in ('DROP TAB','TRUNCATE') and (NOW() - time) < '24 hours';

4. vsql -xc "select * from USER_SESSIONS where user_name='Mike';"
   OR
   grep -e 'sendAuthRequest: user=Mike' $(admintools -t list_db -d EEVDB | grep -e "Catalog Dir" | cut -d: -f2)/vertica.log

5. SELECT * FROM error_messages WHERE user_name='Mike' and message ilike '%permission%' ;
   -[ RECORD 1 ]---+---------------------------------------
   event_timestamp | 2020-05-15 19:42:03.662103-04
   node_name | v_eevdb_node0001
   user_id | 45035996273851018
   user_name | Mike
   session_id | v_eevdb_node0001-290527:0x168ac
   request_id | 0
   transaction_id | 45035996273711676
   statement_id | 1
   error_level | ERROR
   error_code | 16797828
   message | Permission denied for relation my_test
   detail |
   hint |

Here is another example:

WITH mytimes AS (SELECT now() - interval '1 day' AS qstart, now() AS qend)
SELECT user_name,
       CASE WHEN qr.request ilike '%swap_partitions_between_tables%' THEN 'SWAP_PARTITIONS'
            ELSE UPPER(left(qr.request, 5))
       END AS DMLDDL, COUNT(1)
FROM query_requests qr , mytimes
WHERE (    qstart between qr.start_timestamp and qr.end_timestamp
       OR  qend   between qr.start_timestamp and qr.end_timestamp
       OR (qr.start_timestamp < qstart AND qr.end_timestamp > qend)
       OR (qr.start_timestamp > qstart AND qr.end_timestamp < qend)
      )
       AND qr.node_name = (select local_node_name())
GROUP BY 1,rollup(DMLDDL)
ORDER BY 1,3;
user_name |     DMLDDL      | COUNT
-----------+-----------------+-------
 dbadmin   | SWAP_PARTITIONS |     7
 dbadmin   | ALTER                            |    24
 dbadmin   | COPY                             |    25
 dbadmin   | DROP                             |    25
 dbadmin   | CREAT                           |    50
 dbadmin   | WITH                              |    61
 dbadmin   | SELEC                           |   131
 dbadmin   |                                          |   323
(8 rows)

Vertica do not have audit of failed queries, if they failed with permission denied error.

@Sergey_Cherepan_1 Please consider the following:

SELECT VERSION(); --  Vertica Analytic Database v12.0.4-6
CREATE USER user104;
CREATE TABLE dbadmin_table (f1 int);
INSERT INTO dbadmin_table VALUES (1);
COMMIT;

\c - user104
You are now connected as user "user104".
SELECT * FROM dbadmin_table;
vsql:3test.sql:11: ERROR 4367:  Permission denied for relation dbadmin_table

\c - dbadmin
You are now connected as user "dbadmin".
\x
SELECT * FROM error_messages WHERE user_name='user104';
-[ RECORD 1 ]---+---------------------------------------------
event_timestamp | 2023-05-28 17:42:00.412878+03
node_name       | v_eondb_node0001
user_id         | 45035996273851866
user_name       | user104
session_id      | v_eondb_node0001-359523:0x18f25e
request_id      | 0
transaction_id  | 45035996273742525
statement_id    | 1
error_level     | ERROR
error_code      | 16797828
message         | Permission denied for relation dbadmin_table
detail          |
hint            |

Yes, error message is there. SQL text of failed query is missing.
For audit investigations, full text of failed SQL is desirable.

@Sergey_Cherepan_1 : I had already created a new feature request for this scenario in the past. VER-86008