Doctor prescribed amoxiclav+mucinex dm 10ml/6hrs to patient with earlier pneumonia and now atelectasis in both lower lung zones.can mucinex guaifenesin 600 mg/12 hrs tablet for chest congestion be taken instead as mucinex dm syrup is not available?

Mucinex (Guaifenesin) alone does not suppress cough, it is an expectorant (make the phlegm loose). Yes, you can take that if cough does not bother you much. If you want, you can buy Robitussin DM which has the same effects as Mucinex DM. Besides medications, you need to do deep breathing and cough to fight against the atelectasis ( take a deep breath, hold for a second or two then cough out).

The cause of the cough should be considered. Have you had a COVID-19 and Flu screening test as yet? Post nasal drip may be treated with nasal saline rinses which may provide relief. I agree that at night a cough suppressant may be helpful.