"His well-being is very important."
Translation:Su bienestar es muy importante.
Your response was correct as possession can be described with the possessive pronoun or with the "de + pronoun". I reported it as acceptable.
Duo also believes that bienestar in not masculine.
I wrote the same answer. Because I see you have 3 others giving you a thumbs up, is there someone out there that can explain for us why this is not a suitable translation?
My autocorrect put in "wellbeing" and Duo said that it was correct, but that I had a typo and that it should have been "well-being." I think this one is interchangeable, right?
This seems to be a country specific issue: http://grammarist.com/spelling/well-being-wellbeing/
Duolingo should accept both, but you may be better off using the hyphen depending on your audience.