Background information

timestamp at time zone dbtimezone in the dbtimezone are not supported in PolarDB O Edition.
SQL> SELECT timestamp '1970-01-01 0:0:0 -0:0' at time zone dbtimezone FROM dual;
TIMESTAMP'1970-01-010:0:0-0:0'ATTIMEZONEDBTIMEZONE
---------------------------------------------------------------------------
01-JAN-70 12.00.00.000000000 AM +00:00
SQL> 
SQL>  SELECT CURRENT_TIMESTAMP  FROM dual;
CURRENT_TIMESTAMP
---------------------------------------------------------------------------
04-AUG-20 06.04.42.125059 PM +08:00

Solutions

We recommend that you change the time zone to a specific value based on the dbtimezone settings in the ORACLE database, for example, +00:00. The time zone of PolarDB can be modified based on the timezone parameter and can be adjusted at the server level or the session level.
van=> set timezone='+08:00';
SET
van=> select current_timestamp;
        current_timestamp         
---------------------------------- 
04-AUG-20 02:03:27.686021 -08:00
(1 row)
van=> set timezone='UTC';
SET
van=> select current_timestamp;
        current_timestamp
         ----------------------------------
 04-AUG-20 10:03:49.071727 +00:00
(1 row)

For more information, see https://www.postgresql.org/docs/11/functions-datetime.html.