I think I see what the issue is.
is the URL you are trying to activate http://nh.building-stage.com/ ?
I see this is in maintenance mode. This normally means your site returns a 503 (maintenance mode) server response and that usually blocks the license activation.
When your site goes live and the maintenance mode is removed this should work as normal.
ps: don’t use the Lifetime (main) license key, that’s only for admin/renewal purposes. Always use the theme specific keys.