The ChronoPay gateway requires you to create a product in your ChronoPay admin panel before you can use it.
To do this, login at https://clients.chronopay.com/ and click the Setup link on the left. Next to the heading that says List Your Products, click Submit. Once at this page, you will see a button to Add A New Product. Click this and you will then be shown the form. Here you need to select the correct Site ID, enter a Product Name (eg. Hosting Invoice) and Description (eg. Hosting Invoice Payment). You should leave the product type set to Standard and set the price to 1.00. Period, Rebill Price & Rebill Period should all be left blank. You should enter the access url as your websites address and the support email as an email customers can use to contact you. Once you've added this product, you will see the list of all your products again and you now need to get the ID for the newly created product. This will look something like 003095-0001-0001. This is the number you must enter into your WHMCS Gateway Configuration Page.