Existing clustered or cluster-prepared instance = Failed.
Darn! This particular install is now finished, we can't click next and we'll have to start again. This time though we will run the SQL setup slightly different. In a command prompt go to the directory where the setup.exe is situated and run the following command:
Setup.exe /SkipRules=StandaloneInstall_HasClusteredOrPreparedInstanceCheck /Action=Install
This will ignore the clustered instance check and allow you to install SSRS as normal. Please note the process must be repeated on any other nodes in the cluster. One final point if you are using a report database on the cluster, you must specify the clustered instance name, not the node name.