Why WPForms plugin doesn't send data to my salesforce account?

I'm using WP plugin called WPForms (Elite version) and I have faced the following problem. I'm trying to integrate WPForms with Salesforce (I also have an upgraded version of Salesforce account).

However, when I send data using my new form, it doesn't reach my Salesforce account (e.g: I've sent full 'Contact' form, but my list of contacts in Salesforce is empty). I've gone through this article three times but still no luck: https://wpforms.com/docs/how-to-install-and-use-the-salesforce-addon-with-wpforms/

What am I doing wrong?

P.S: 'Salesforce' addon for WPForms is already installed.