DBMS_STATS: GATHER_STATS_JOB encountered errors.  Check the trace file.
Errors in file /desdb_j000_26964.trc:
ORA-20011: Approximate NDV failed: ORA-29913: error in executing ODCIEXTTABLEOPEN callout
KUP-11024: This external table can only be accessed from within a Data Pump job.

Error analysis：
The external table is not deleted normally，There is still information left in the data dictionary，The table does not actually exist.
An error is reported when the database runs the collect statistics job to collect these external tables.

Solution:

1. Find the object starting with ET：

select owner,
       object_name,
       object_type,
       status,
       to_char(created, 'yyyy-mm-dd hh24:mi:ss') created,
       to_char(last_ddl_time, 'yyyy-mm-dd hh24:mi:ss') last_ddl_time
  from dba_objects
 where object_name like 'ET$%';

2. Confirm that it belongs to the external table generated by dump

select owner,table_name,default_directory_name,access_type from dba_external_tables order by 1,2;

3. Delete

drop table SYSTEM.ET$02D805830001;

drop table SYSTEM.ET$03B700030001;