、。
Sadly, my father won't be able to attend the meeting.
Literal
Sad [to the fact that-ことに], father [topic-は] that meeting [to-には] attend-cannot.
出られない is the potential negative of 出る — here 出る means 'to attend, take part in,' so the negative potential gives 'cannot attend.' 悲しいことに ('sadly, regrettably') frames the outcome as an emotional reaction; it joins a small family of [adjective]+ことに openers like 嬉しいことに ('happily') and 驚いたことに ('surprisingly').