"Required parameter missing or invalid: items" Error when adding to cart

[EricCarey286]

I use Cart.js to upsell warranty products on my cart page. I found a solution on Shopify's site using an upsell.liquid file and JavaScript. I am fairly new to Shopify and not sure how everything works. The products get added underneath the car with a button that says add to cart but the button does nothing. When i check the console I see this error: POST : https://wi-plum-test.myshopify.com/cart/add.js Status: 404 Not Found. When I look at '/cart/add.js' I see this message "{"status":"bad_request","message":"Parameter Missing or Invalid","description":"Required parameter missing or invalid: items"}"

Any help would be greatly appreciated

Here is the upsell.liquid file and custom.js files:
upsell.liquid:

{% comment %} Create liquid variable for collection {% endcomment %}
{% assign upsellCollection = 'upsell-products' %}
<div class="c-upsell js-upsell">
    {% comment %} Use Product ID as Data Attribute which we will use for Add to Cart Event {% endcomment %}
    {% for product in collections[upsellCollection].products %}
    <div class="c-upsell__product js-product" data-product-id="{{ product.id}}">
        <div class="c-upsell__productImage">
            {% comment %} Fetch Product Images{% endcomment %}
            {% for image in product.images %}
            <img class="c-upsell__innerImage" src="{{ image | img_url: '586x' }}" alt="Product Img" />
            {% endfor %}
        </div>
        {% comment %} Fetch Product title & Price {% endcomment %}
        <h2 class="c-upsell__productTitle">{{ product.title }} </h2>
        <p class="c-upsell__productPrice"> {{ product.price }}</p>
        {% comment %} Add to Cart Button {% endcomment %}
      	<a href="#" class="c-upsell__atc js-atc">Add to Cart</a>
    </div>
    {% endfor %}
</div>
{% comment %} Now we're done with Liquid code and collection will be appeared on where we add it {% endcomment %}

custom.js:

const upsellProductArr = [];
function upsellProduct() {
	// Create an empty Array in which we'll be pushing our product ID 
	
	//get the Product ID from Attribute
	const productID = document.querySelector('.js-product').getAttribute('data-product-id');
 
	// Now push this product ID to Array
	upsellProductArr.push(productID);
  
	// Console.log to check you're pushing correct ID into array
	console.log(upsellProductArr);
 
}

// Let's Find our ATC button
const button = document.querySelector('.js-atc');
	// Now Add click event use our Array to add to cart the Product 
button.addEventListener('click', event => {
  
	upsellProduct();

	// This is CartJS API - Attaching link for reference 
	 CartJS.addItem(upsellProductArr[0], quantity = 1);
		
});
});