Не архивные таблицы такой же структуры как и архивные:
create table tasks_arch (task_id number not null,
                         task_name varchar2(200) not null,
                         start_date date,
                         stop_date  date,
                         description varchar2(200),
                         manager_id number);
create table workflow_arch(task_id number not null,
                           employee_id number not null);

declare
  p_start_date date := to_date('11.10.2019', 'dd.mm.yyyy');
  lr_tasks tasks%rowtype;
begin
  for rec_all_tasks in (select rowid, task_id
                        from tasks t
                        where t.start_date < (p_start_date)+1
                       )
  loop
    begin
      SavePoint sp_before_tasks;

      select *
      into lr_tasks
      from tasks t
      where t.rowid = rec_all_tasks.rowid
      for update nowait;

      begin

        insert into tasks_arch(task_id,
                               task_name,
                               start_date,
                               stop_date,
                               description,
                               manager_id)
        select task_id,
               task_name,
               start_date,
               stop_date,
               description,
               manager_id  
        from tasks t
        where t.task_id = lr_tasks.task_id;

        insert into workflow_arch(task_id, employee_id)
        select task_id, employee_id
          from workflow w
         where w.task_id = lr_tasks.task_id;

        delete from workflow w
        where w.task_id = lr_tasks.task_id;

        delete from tasks t
        where t.rowid = rec_all_tasks.rowid;

      exception
        when no_data_found then

          rollback to sp_before_tasks;
          continue;
      end;
      
    exception
      when others then
        rollback to sp_before_tasks;
    end;

  end loop;

  commit;
end;