SSRS The permissions are insufficient for performing this operation

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:

1. Start Internet Explorer using "Run as Administrator"
2. Open http://localhost/reports
3. Go to properties tab (SSRS 2008)
4. Security->New Role Assignment
5. Add DOMAIN/USERNAME or DOMAIN/USERGROUP
6. Check Report builder

According to the article How to fix Reporting Services permissions are insufficient for performing operation (rsAccessDenied):

1. Make sure you have access configured to the URL http://localhost/reports using the SQL Reporting Services Configuration. To do this:
   1. Open Reporting Services Configuration Manager -> then connect to the report server instance -> then click on Report Manager URL.
   2. In the Report Manager URL page, click the Advanced button -> then in the Multiple Identities for Report Manager, click Add.
   3. In the Add a Report Manager HTTP URL popup box, select Host Header and type in: localhost
   4. Click OK to save your changes.
2. Now start/ run Internet Explorer using Run as Administrator.
   • NOTE: If you don't see the 'Site Settings' link in the top left corner while at http://localhost/reports it is probably because you aren't running IE as an Administrator or you haven't assigned your computer's 'domain\username' to the reporting services roles, see how to do this in the next few steps.
3. Then go to: http://localhost/reports (you may have to login with your Computer's username and password)
4. You should now be directed to the Home page of SQL Server Reporting Services here: http://localhost/Reports/Pages/Folder.aspx
5. From the Home page, click the Properties tab, then click New Role Assignment
6. In the Group or user name textbox, add the 'domain\username' which was in the error message
   • *(in my case, I added: DOUGDELL3-PC\DOUGDELL3 for the 'domain\username', in your case you can find the domain\username for your computer in the rsAccessDenied error message).
7. Now check all the checkboxes; Browser, Content Manager, My Reports, Publisher, Report Builder, and then click OK.
8. Your domain\username should now be assigned to the Roles that will give you access to deploy your reports to the Report Server. If you're using Visual Studio or SQL Server Business Intelligence Development Studio to deploy your reports to your local reports server, you should now be able to.