Email Exists ERROR into Ajax registration form

Why are you making work hard for yourself? wp_create_user already checks if the email/login exists, which is also why your code is “failing” – $user_id will already be a WP_Error, so your elseif ( is_wp_error( $newerrors ) ) never fires.

All you need is:

$user_id = wp_create_user( $sanitized_user_login, $user_pass, $user_email, $capa );

if ( is_wp_error( $user_id ) ) {
    wp_send_json( array(
        'loggedin' => false,
        'message'  => $user_id->get_error_message(),
    ) );
}

exit;

Also note I’ve used the WordPress helper wp_send_json, which is the correct way of sending JSON data back to the client.