Hi ilaria,
Effective October 2018, access to the first included portal for the tenant requires the purchase of a minimum of 10 Enterprise user licenses of Dynamics 365 Customer Engagement Plan, Dynamics 365 Customer Engagement applications (Sales Enterprise, Customer Service Enterprise, Field Service or Project Service Automation) or a combination.
Please refer to page 3 of this Microsoft Dynamics 365 Licensing Guide:
[View:https://mbs.microsoft.com/Files/public/365/Dynamics365LicensingGuide.pdf:320:50]