can I cash a deceased persons check if I have the death certificate

I assume you mean a check made out TO the decedent; there is no legal reason you can't cash a check FROM a deceased person (although you may run into practical difficulties, such as the account being frozen).  However, you can't cash a check made out to the deceased person, as it is an asset belonging to the estate.  Only the person or persons appointed by the Court as administrator/administratrix (if no will) or executor/executrix (if there is a will) of the estate can cash the check, the proceeds of which become part of the estate and distributed along with all other assets of the estate.