Redirect users without access to a single bbPress forum to the Membership Levels page.

redirect_bbpress_no_access_levels_page.php

<?php
/**
 * Redirect users without access to a single bbPress forum to the Membership Levels page.
 *
 */
function redirect_bbpress_no_access_levels_page( $redirect_to, $forum_id ) {
	global $pmpro_pages;

	$redirect_to = get_permalink( $pmpro_pages['levels'] );
	
	return $redirect_to;
}
add_filter( 'pmprobbp_check_forum_redirect_url', 'redirect_bbpress_no_access_levels_page', 10, 2 );