Redirect based on referer using Advanced Custom Fields

Try the template_redirect hook

function portal_protection() {

    if ( get_field('enable_portal_protection') && !current_user_can('administrator') ) {
        $referring_url = get_field('referring_url');
        $redirect_url = get_field('redirect_url');
        $referer = $_SERVER['HTTP_REFERER'];

        if ( $referer != $referring_url) {

            wp_redirect( $redirect_url );
            exit;
        }
    }
}
add_action( 'template_redirect', 'portal_protection' );

Also use wp_redirect. it’s the WordPress way of redirecting.