Don't versus can't

1. I'm listening to music.

my teacher told me that you don't say " listen at" , cause listen to is a set phrase.

my question is why did teacher say "don't say" instead of saying" can't say "?

2."you don' write bestseller, cause you despise emotions of people", John said.

my initial reaction to this sentence is you don't write something means something are not done and finished, now that you didn't write it, how it become a bestseller?

However, John's original meaning is you wrote a book , while the book you wrote is not good enough to impress people, so it hasn't become popular.

So I'm confused, what's slight distinction between "you don't write" and "you can't write bestseller"??