In normal circumstances addition of any service SSAS ,SSRS or DB engine is not allowed on existing installation and addition of these would need a named Instance installation.
This issue occurred somewhere in 2013 on a SQL server 2008 R2 hosted on Windows server 2008 R2.
However this was not normal circumstance and the client wanted us to add SSRS service to already existing SQL Server installation. This is were it got interesting as when you run SQL installation wizard it will fail during Installation rules check. see image
Error implies that SQL Server is already installed and clustered on the server.
To overcome this error we have a option with command line install that can be used here, open an elevated command prompt window , browse to directory where you have placed your setup files. now Run SQL Server setup.exe with following parameters
Setup.exe /SkipRules=StandaloneInstall_HasClusteredOrPreparedInstanceCheck /Action=Install
Press enter and it will take you to the initial setup and on installation rules page you will see below screenshot
Setup will bypass that check and Voila you are good to proceed with the install.
Same process should be repeated on passive node as well , so that both clustered nodes have same binaries