It'll never fly:

It will never work!; It will never be approved!

不可行 行不通 √

例句： 

Your design for a new electric automobile is interesting, but it'll never fly!

你的新電動汽車設(shè)計很有趣，但它不可行??！

Now or never:

At the current time or not at all. 

機不可失 √

例句：

It's now or never, so make up your mind.

機不可失，下決心吧。

A barking dog never bites:

One who regularly makes angry or threatening statements rarely acts upon them.

會叫的狗不咬人; 虛張聲勢的人 √

例句：

Mr. Stewart may yell a lot, but I doubt he'll do anything to you — a barking dog never bites.

斯圖爾特先生可能會大喊大叫，但我覺得他不會對你怎么樣的，他只是一個虛張聲勢的人。

Song for you（片尾曲）

Charlie Puth - Look At Me Now