Force coupon for certain product in Woocommerce

[ChuckSA]

I'm trying to force the user to apply a coupon for one specific product/category only.

I found the following code  but am having endless trouble getting it to run. I suspect it's old code and am unsure what to change. It's supposed to go in the functions file, so I've been using the Code Snippets plugin to run it

// Force Coupon codes for Woocommerce
add_action('woocommerce_check_cart_items', 'mandatory_coupon_code');
function mandatory_coupon_code()
{
        // set Here your categories IDs, slugs or names
        $categories = array(18,19,87);
        $found = false;

        // Loop through cart items
        foreach ( WC()->cart->get_cart() as $cart_item ) 
            if( has_term( $categories, 'product_cat', $cart_item['product_id'] ) ){
                // Product category found in cart items
                $found = true;
                // Exit from loop
                break;
            }
        }
        $coupons = WC()->cart->get_applied_coupons();

        // The Notice is displayed for that product categories when no mandatory coupon has been entered
        if( count( $coupons ) > 0 && $found )
            wc_add_notice( __( 'A coupon code is mandatory for this product.', 'woocommerce' ), 'error' );

I will truly appreciate any help I can get here.