Can i post a UI UX feedback challenge like http://challenge.nexus360.mobi/ here?
I don't think so. Something like this would really belong in the chat rooms but you're not able to even access those until you get 20 rep.
In the tour of this site they say that "UX Stack Exchange is a question and answer site ...This site is all about getting answers. It's not a discussion forum. There's no chit-chat."
You're not really asking a direct question to get help with UX problems. I'd take a look at the tour in the Help section in the top navigation bar to get a better feel for what you can and can't do here.
That appears to be a spec-work site? That's usually frowned upon in general as it's not good for the industry, the designer, or the client.