To enable AUTOTRACE feature in your session, you not only need to create PLAN_TABLE table, but also need PLUSTRACE role, which is not native in database and needs to be installed.
To install PLUSTRACE, just execute $ORACLE_HOME/sqlplus/admin/plustrce.sql by SYS.
SQL> show user
USER is "SYS"
The following is the execution log of $ORACLE_HOME/sqlplus/admin/plustrce.sql.
SQL> drop role plustrace;
drop role plustrace
ERROR at line 1:
ORA-01919: role 'PLUSTRACE' does not exist
SQL> create role plustrace;
SQL> grant select on v_$sesstat to plustrace;
SQL> grant select on v_$statname to plustrace;
SQL> grant select on v_$mystat to plustrace;
SQL> grant plustrace to dba with admin option;
SQL> set echo off
Now, PLUSTRACE has been installed.