To sell the products from WooCommerce trought Plug&Pay you need a checkout page. Before you make the checkout page there first need to be created a Dummy product.
To do this go in your Plug&Pay account to Product and click on Create Product.
Fill in the name off the product and click Save.
After that you will see the page with product information. at Properties make sure Physical product is turned on. Go the the tab Advanced and empty the SKU field. after that click on Save Changes.
Now the checkout page will be created. Klik on the navigation menu on Checkouts. on that page you can press Created checkout.
In the pop-up screen fill in a name for the checkout. Choose the dummy product and press save
At Settings > Payments you can put your Plug&Pay on testmodes. If your Plug&Pay account is in testmode, you can order something without accualy paying for it. Do this to make sure everything works correctly.
Pay Attention: If testmode is turned of, this is for all checkouts. so customers cant order while it is testmode.