Require user to have a prerequisite level before purchasing a restricted level

my_pmpro_enforce_prerequisite_level.php

<?php // copy from below
/*
 * Require user to have a prerequisite level before purchasing a restricted level.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 * 
 */

function my_pmpro_enforce_prerequisite_level( $okay ) {

	$prerequisite_levels = array( 1, 2 ); // at least one of these levels is required to buy the restricted level
	$restricted_level = 3; // Level that requires a prerequisite level to be purchased

	$pmpro_level = pmpro_getLevelAtCheckout(); // checkout level
	
	if ( $okay && $pmpro_level->id == $restricted_level && ! pmpro_hasMembershipLevel( $prerequisite_levels ) ) {
		pmpro_setMessage( 'You must be a member of level 1 or level 2 to purchase level 3.', 'pmpro_error' );
		$okay = false;
	}

	return $okay;
}
add_action( 'pmpro_registration_checks', 'my_pmpro_enforce_prerequisite_level' );