Why I cannot get execution plan from AWR Repository?

SQL "select * from table(dbms_xplan.display_awr('sql_id')); can query the execution plan in AWR Repository.

My command is:

SQL> select count(*) from dual;

  COUNT(*)

----------

         1

SQL> select sql_text, sql_id, version_count, executions from v$sqlarea where sql_text like 'select count(*) from dual%';

SQL_TEXT                      SQL_ID                        VERSION_COUNT EXECUTIONS

------------------------------------ ----------------------------------- --------------------------- --------------------------------

select count(*) from dual  4m94ckmu16f9k             1                         1

create snapshot for AWR:

SQL> exec dbms_workload_repository.create_snapshot();

PL/SQL procedure successfully completed.

flush cache:

SQL> alter system flush shared_pool;

System altered.

SQL> select sql_text, sql_id, version_count, executions from v$sqlarea where sql_text like 'select count(*) from dual%';