this error occur when I run the SSRS report. Here are to solution :

1. on your windows server that contains SQL Server, go to C:\Program Files\Microsoft SQL Server, then do find the MRS for your environment.

 
2. find rssrvpolicy.config

 
3. replace PermissionSetName=”Execution” on class=”UnionCodeGroup” with PermissionSetName=”FullTrust”

source : https://msdn.microsoft.com/en-us/library/ms155866.aspx