Prevent upsell product from being purchased on its own

thp_remove_upsell.php

<?php
function thp_remove_upsell ($cart) {
	
	$main_product_ID = 38; //CHANGE TO THE MAIN PRODUCT ID
	$upsell_product_ID = 39; //CHANGE TO YOUR UPSELL PRODUCT ID
	
	$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;
		}
		else if ( $cart_item['product_id'] == $upsell_product_ID ) {
			$has_upsell = true;
			$upsell_cart_key = $cart_item_key;
		}

	}
	
	if ( $has_upsell && !$has_main_product ) {
		$cart->remove_cart_item( $upsell_cart_key );
	}
	
}
add_action( 'woocommerce_before_calculate_totals', 'thp_remove_upsell' );