To fail to do something one intended to do; to miss or botch (used after a verb stem or with nouns + する).
予約を忘れてしまい、チケットを取りし損ねた。
I forgot to make a reservation and missed getting the tickets.
重要な書類を提出し損ねると困るから、今すぐ確認しておいてください。
It would be a problem if you failed to submit the important documents, so please check them right away.
彼は電車に乗り遅れて会議に出席し損ねた。
He missed the train and failed to attend the meeting.