You can't use Nested Table declared in anonymous PL/SQL block in SQL context.

nt.pls

declare
  TYPE row_nt IS TABLE OF number;
  a row_nt;

  cursor a_cur is
    select level from dual connect by level <= 100;
  
  n number;

begin

  open a_cur;
  fetch a_cur bulk collect into a;
  close a_cur;

  dbms_output.put_line(a.COUNT);

  select count(column_value)
    into n
    from table(a); -- ERROR HERE


end;
/