Did you get an e-mail declining it, or does it just not let you past the sign-up page with that nick? If it is the second, it is due to a bug with the validation script that often prevents valid handles from being requested; you may need to try some alternate names until you find something that works.
Sorry, I have no way to do that. I think Khan has been able to help people with this in the past, though. In case he does not check this thread, you can contact him directly at firstname.lastname@example.org and include the handles that you have tried so he can narrow down the problem.