Power Bi On Prem

There are currently only two ways of purchasing Power BI Report Server.

1) Purchase Premium capacity on Power BI. Then you are entitled to also run the same number of cores in an on-prem Power BI Report Server instance

2) purchase SQL Server Enterprise WITH Software Assurance.

So if you don't have Software Assurance I believe the only other way to license Power BI Report Server is to purchase a subscription for Power BI Premium. (and you don't install it onto Reporting Services, you would install it instead of Reporting Services, it is a superset of Reporting Services, it does all the same things and adds support for pbix files)