連詞
表示一樣嘢、一件事嘅缺失或者問題所在 but, with the meaning of "unfortunately", the following phrase usually describes the cause or problem; used to indicate a critical problem that negates the positive aspects of the subject
佢好努力，又上進，弊在成日得罪人，所以冇得升職。
佢好努力，又上进，弊在成日得罪人，所以冇得升职。
He is hardworking and diligent, unfortunately, he often offends people, so he never gets a promotion.
Definitions (Wiktionary)
adv, Cantonese
unfortunately (syn.) 不幸, 不巧
呢件衫个款都几靓，弊在冇件啱身。
呢件衫个款都几靓，弊在冇件啱身。
Nei1 gin6 saam1 go3 fun2 dou1 gei2 leng3, bai6 zoi6 mou5 gin6 ngaam1 san1. This shirt looks quite nice. Unfortunately, none of them fit.
Examples (粵典–words.hk)
Cantonese
佢好努力，又上进，弊在成日得罪人，所以冇得升职。
佢好努力，又上進，弊在成日得罪人，所以冇得升職。
He is hardworking and diligent, unfortunately, he often offends people, so he never gets a promotion.