oracle – Facing error “cannot fetch plan for SQL_ID” while trying to capture the execution plan

I’m trying to capture the execution plan of my query on PL/SQL developer as you can see below :

select  *
  from vmi_dimcustomer t1
 inner join vmi_factcustomer t2
    on t1.customer_num = t2.customer_num  ;
Select plan_table_output from table(dbms_xplan.display_cursor(null,null,'basic'));

but I receive this note :

  SQL_ID  9m7787camwh4m, child number 0
  begin :id := sys.dbms_transaction.local_transaction_id; end;
  NOTE: cannot fetch plan for SQL_ID: 9m7787camwh4m, CHILD_NUMBER: 0
    Please verify value of SQL_ID and CHILD_NUMBER; 
    It could also be that the plan is no longer in cursor cache (check v$sql_plan)

What am I doing wrong here?
I searched here and the answer I got was to "set serveroutput off" . Put I can not do this on PL/SQL developer.

Thanks in advance.