Scheduled Maintenance: We are aware of an issue with Google, AOL, and Yahoo services as email providers which are blocking new registrations. We are trying to fix the issue and we have several internal and external support tickets in process to resolve the issue. Please see: viewtopic.php?t=158230
I'm not sure what the problem is, but it's possible that Nix doesn't properly install itself on Debian. Or maybe the installation instructions are incomplete.
I found a possible clue, though:
At this stage, nix is properly installed on your system, but some extra configuration is required, especially to activate the “profiles” feature.