The SSRS service itself is not cluster aware. The only things you can cluster with SSRS are:

- The ReportServer databases

- The data sources

- The IPs that the ReportServer web service and the Report Manager interfaces are on

If you want HA for SSRS the preferred way is to setup multiple SSRS servers in a scale-out deployment with a load-balanced configuration and have the Report Server databases on a cluster (or AlwaysOn AG in newer versions if preferred).

High Availability (Reporting Services)