when upgrading a database from 12cR1 to either 12cR2 or 18c , you will notice when querying dba_users view a new column added: LOCAL_TEMP_TABLESPACE
local temp tablespace is most probably used for RAC.
you need to ensure that its not assigned to “SYSTEM” tablesapce, to check this:
SQL> select username from dba_users where local_temp_tablespace=’SYSTEM’;
if there any….you can set it to temp tablespace as follows:
SQL> alter user C##ADAM LOCAL TEMPORARY TABLESPACE TEMP;