Prevent upsell products from being purchased without the main product

thp-remove-upsells.php

<?php
function thp_remove_upsells ($cart) {
	
	$main_product_ID = 38; //CHANGE TO THE MAIN PRODUCT ID
	$upsell_product_IDs = array( 39, 40 ); //CHANGE TO YOUR UPSELL PRODUCT ID's
	$upsell_cart_keys = array();
	
	$has_main_product = false;
	$has_upsell = false;
	
	foreach( $cart->get_cart() as $cart_item_key => $cart_item ) {
		if ( $cart_item['product_id'] == $main_product_ID ) {
			$has_main_product = true;
		}
		
		foreach ($upsell_product_IDs as $upsell_product_ID) {
			if ( $cart_item['product_id'] == $upsell_product_ID ) {
				$has_upsell = true;
				$upsell_cart_keys[] = $cart_item_key;
			}
		}
	}
	
	if ( $has_upsell && !$has_main_product ) {
		foreach ($upsell_cart_keys as $upsell_cart_key) {
			$cart->remove_cart_item( $upsell_cart_key );
		}
	}
	
}
add_action( 'woocommerce_before_calculate_totals', 'thp_remove_upsells' );