If you are locally logged on the SSRS server as an admin and when opening Report Manager you receive an error like this:
User ‘…’ does not have required permissions. Verify that sufficient permissions have been granted and Windows User Account Control (UAC) restrictions have been addressed.
then please apply the steps this article: https://technet.microsoft.com/en-us/library/bb630430(v=sql.120).aspx to manage SSRS locally.
Note: This OS limitation is NOT in effect when accessing SSRS remotely. You should be able to manage SSRS remotely without doing the steps mentioned in the article.