What does low pneumococcal antibodies mean?blood showed good immune system except that? can it cause chronic sinusitis and bronchitis?

It means exposure: You've been exposed to Pneumococcus, not necesarily presntly infected

Yes: I assume this is a antibody level measured after the pneumococcal vaccine. If so, it means your immune system is not responding to the bacterial proteins in the normal way. Patients who mount a weak response to bacterial infections may be at higher risk for recurrent respiratory tract infections such as sinusitis and bronchitis.