It’s because of lack of privilege for the user you are running the report builder, just give that user or a group a privilege to run report builder. Please visit this article
Or for shortcut:
- Start Internet Explorer using “Run as Administrator”
- Open http://localhost/reports
- Go to properties tab (SSRS 2008)
- Security->New Role Assignment
- Add DOMAIN/USERNAME or DOMAIN/USERGROUP
- Check Report builder