。
Due to illness, I was unable to visit today.
Literal
Illness [because-のため] today [topic-は] visiting [could-not-うかがうことができませんでした].
うかがう is the humble form of 行く/訪問する — 'to visit (humbly).' ことができませんでした is the polite negative potential. のため gives the reason. This is polite/humble language appropriate for apologizing to a superior for a missed visit.