Which word is correct 秒 or 妙
「美秒」is not a word. It also doesn't make any sense.

In this context, 美妙 means "awesomeness", "wonderfulness". Noun or adjective isn't that important in Chinese as long as it makes sense.


  天下許多事，你都可以盡性地去嚮往，嚮往給你帶來的無窮美妙其實已經足夠了。


"There are a lot of things that you can long for in this world. The unlimited wonderfulness you get when dreaming about it is actually already enough. "


  這美妙，就是在於它不真實的一面，一旦實現，便要大打折扣。


"This wonderfulness comes from the fact that it is not real. Once you reached the goal of having the things you longed for, that feeling will get discounted (will no longer be this wonderful)."

These sentences sound like they are from a novel. The full text I found


  人生在世，有許多解不開的結，也有許多莫須有的煩惱。因為天下許多事情，你都可以盡情地去嚮往，嚮往給你帶來的無窮美妙，其實已經足夠了。不要為嚮往的沒有實現而遺憾，實現了也許更遺憾！
  
  人世間有許多事情，只有一直嚮往著才是最美好的。這美妙就是在於他有不真實的一面，一旦實現便要大打折扣。如果真的讓他實現了，最好只有一天，一夜，一會兒，或是短短的一瞬間，真的不能再長了。


have some deep meaning. Feels like it's something that young girls like to read in a coffee shop. It's basically saying you don't need to reach your goal to be happy. The journey is often more enjoyable than the short moment of you reaching the goal.



PS: Some people are saying this is not a good text and I don't see why. This style of writing is quite common in novels.