If your AUD$ table is in SYSTEM and SYTEM tablespace, Then it is advised to move the AUD$ to a dedicated tablespace.

Use below steps to move AUD$.

select owner,segment_name,segment_type,tablespace_name,bytes/1024/1024 from dba_segments where segment_name='AUD$'

OWNER   SEGMENT_NAME  SEGMENT_TYPE       TABLESPACE_NAME                BYTES/1024/1024
------- ------------- ------------------ ------------------------------ ---------------
SYS     AUD$          TABLE              SYSTEM                                      16

Use the dbms_audit_mgmt to move the tablespace .

BEGIN
DBMS_AUDIT_MGMT.SET_AUDIT_TRAIL_LOCATION(audit_trail_type => DBMS_AUDIT_MGMT.AUDIT_TRAIL_AUD_STD,
audit_trail_location_value => 'AUDIT_DATA');
END;
/

PL/SQL procedure successfully completed.

check whether tablespace has been moved from system to AUDIT_DATA or not.

select owner,segment_name,segment_type,tablespace_name,bytes/1024/1024 from dba_segments where segment_name='AUD$'

OWNER   SEGMENT_NAME  SEGMENT_TYPE       TABLESPACE_NAME                BYTES/1024/1024
------- ------------- ------------------ ------------------------------ ---------------
SYS     AUD$          TABLE              AUDIT_DATA                            16