UsersWP shows “Heads Up! User registration is currently not allowed.” on the register page when WordPress is set to refuse new accounts. UsersWP follows the WordPress setting, so that is where you turn registration on.

Single site

1. In wp-admin, go to Settings > General.
2. Next to Membership, tick Anyone can register.
3. Check New User Default Role. Subscriber is the usual choice.
4. Click Save Changes and reload your register page.

Multisite

On a network the setting is in Network Admin > Settings. Under Allow new registrations, choose User accounts may be registered, or Both sites and user accounts can be registered. The other two choices, Registration is disabled and Logged in users may register new sites, both block new user accounts.

The UsersWP setup wizard also shows this option on its General Settings step while registration is turned off.

If registration is on and people still cannot sign up, see Why can my users not register?.