If you happen to be using a theme that relies on the Membership plugin, you may run across an issue where your register form doesn’t allow registering a new user. You’ll see this message displayed:
This means that you have not activated registration for all users.
To activate registration for all users, simply navigate to Settings > General and under Membership tick the Anyone can register box. Once you do that, save your new settings.
Now your registration form is enabled for anyone who visits your websites.