The last and very important step is to add the API credentials from your Paypal Business Account into your WooCommerce Plugin Settings.

This step is essential!

> First go to your paypal account and get the API Credentials:

Tools > all tools

> Click on API Access and follow

> Click manage API Credentials, copy and past Username, Password and Singature into your wordpress admin area

RodneyJR provided me with the perfect link with a unfailable step by step instruction. I recommend you to follow this:

https://docs.woocommerce.com/document/paypal-express-checkou...

Now you are all set :)