When you do an explain plan, then the appropriate DBMS_XPLAN api is DBMS_XPLAN.DISPLAY.
When you do an actual execution - i.e. not with explain plan - then DBMS_XPLAN.DISPLAY_CURSOR is the appropriate api.
Troubleshooting for DBMS_XPLAN.DISPLAY_CURSOR
NOTE: cannot fetch plan for SQL_ID: 3pb80q4uks11n, 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$sql_p
grant select ON v_$session TO &&1;
grant select on v_$sql_plan to &&1;
grant select on v_$sql to &&1;
set serverouput off;