(phrase) If you let the chance slip away, you are not going to have a second one; opportunity knocks but once
Definitions (粵典–words.hk)
語句
勸人把握最後機會,唔係就冇下次 used to advise someone that he or she should seize the last chance, i.e. "it's now or never"; literally: after passing Suzhou, cannot travel by boat
呢批貨最後一水喇,要買趁早,唔係蘇州過後冇艇搭。
呢批货最后一水喇,要买趁早,唔系苏州过后冇艇搭。
ni1 pai1 fo3 zeoi3 hau6 jat1 seoi2 laa3, jiu3 maai5 can3 zou2, m4 hai6 sou1 zau1 gwo3 hau6 mou5 teng5 daap3. This is the last batch of this product, you'd better buy it quickly, or you will miss the last chance.
Definitions (Wiktionary)
phrase, Cantonese, idiomatic
there is no second chance after missing this opportunity