I put "his bag has an interesting content" sounds more english. So why did I get incorrect?
It is a more English translation - the one given isn't English at all.
Another possibility would be "His bag has interesting contents" but I knew that wouldn't be accepted either.
I guess sometimes on here we just have to accept it won't take correct translations and just translate things literally, even if our answer isn't English as we native speakers know it.
I am a native English speaker and can tell you that you can never have "an interesting content." Content (in media) is like water, it is uncountable and therefore can never be singular. You can, however, have "a piece of content," (like "a drop of water"), when referring to media.
When we are referring to a mass of items, like those in a bag, we say "contents," with an s. Contents cannot be singular either, because when it is singular, you specify the item or say something like, "His bag has something interesting in it."
The correct English translation for this translation exercise is "His bag has interesting contents."