SQL Server account permissions for New ReportServer database from wizard
@Kin- Thanks for your answer. I was able to refer to this from your answer and get it resolved.
https://social.technet.microsoft.com/Forums/en-US/7541b1ce-aff8-4385-9544-8f9422d01ba7/cannot-alter-the-role-rsexecrole-because-it-does-not-exist-or-you-do-not-have-permission

The fix is weird- this basically covers all the permissions that are collectively included in the RSExecRole but somehow works when executed invidually. Weird!!