Positively I'm using completed or success (I don't want to use successful) together with the word challenge.
Now I'm looking for the negative equivalent. However I can't use unsuccess because that doesn't exist. (And not unsuccessful either because I don't want to use successful)
I don't want to use incomplete either because that implies the challenge is still open for completion and I don't want to imply that.
I don't want to use failure either because some people could read that as "(you are a) failure".