He couldn't attend the meeting because of illness.
Literal
Illness [because-のために] he [topic-は] that meeting [to-に] attending [could-not-できなかった].
のために with に is interchangeable with のため for causal meaning. 出席できなかった uses the potential negative of the suru-verb 出席する ('to attend'). A straightforward cause-and-inability sentence.