Follow the below steps:
1. install Power BI Desktop associated with Power BI Server.
2. Open Power Bi Desktop, Click on Get Data.
3. Click on Analysis Services
4. Enter server and password details in the Dialog Box and click OK.
5. You are now connected to the server from Desktop.
6. Create a report as required and go to Save as and select the Power BI Report Server.
You can now find the report in server (web portal).