Opportunity knocks but once.

良机难再。

《新英汉大辞典》

Opportunity knocks but once, so we must make the most of it.

机不可失，失不再来，所以一定要把握时机。

youdao

We believe in the old saying, "Opportunity knocks but just once," so we always welcome it and devote all our spirit and efforts to it.

我们相信，在这个古老的谚语说：“机不可失，但只有一次”，所以我们一直欢迎它，把我们所有的精神和努力。

youdao