1. Register a new account with Office 365 Business Premium Trial and wait until SharePoint/OneDrive finished
2. Go to Purchase Service: https://admin.microsoft.com/AdminPortal/Home#/catalog and search for PowerBI
3. Register Power BI
4. Verify PowerBI (free) in your Products & Services
5. Make sure all users has Power BI (free) license, and they should be able to access to their mailbox or onedrive.