If you look at https://joindiaspora.com/statistics, you'll actually notice that "Registrations" are "Closed". I guess that means that this specific "pod" is full and you need to find a different "pod"?
Here is a tool to view info about pods: https://podupti.me/
On theis diaspora wiki page, the GIANT sign up link will direct you to a random pod: https://wiki.diasporafoundation.org/Choosing_a_pod