Permissions granted are insufficient for this operation
Report builder works on different permissions than the raw SQL Server permissions which enable you to log into the SQLOS and execute code against the DB engine.  Report builder needs you to identify users in other areas.

Microsoft has some documentation on the matter here.  If you are running native mode (non sharepoint) you can check:

https://msdn.microsoft.com/en-us/library/ms365173.aspx

You will need to add users which can be found here on MS docs:

https://msdn.microsoft.com/en-us/library/ms156034.aspx

To add a user or group to a system role:

Start Report Manager (SSRS Native Mode).

Click Site Settings.

Click Security.

Click New Role Assignment.

In Group or user name, enter a Windows domain user or group account in this 
format: \. If you are using forms authentication or custom 
security, specify the user or group account in the format that is correct for your deployment.

Select a system role, and then click OK.

Roles are cumulative, so if you select both System Administrator and System 
User, a user or group will be able to perform the tasks in both roles.
Repeat to create assignments for additional users or groups.