Error while configuring custom security on power bi report server

I assume you are deploying custom assembly.

Microsoft docs says "Custom data, delivery, rendering, and security extensions in Reporting Services must run under the FullTrust permission set. Work with your system administrator to add the appropriate code group and membership conditions for your Reporting Services extensions."

If so, try editing rssrvpolicy.config file located at

x:\Program Files\Microsoft Power BI Report Server\PBIRS\ReportServer

to configure permission for this assembly.

You will see PermissionSetName="Execution" property under CodeGroup tag. This is default for every custom assembly added. If you change it to FullTrust (beware its case sensitive), all custom assemblies will run under "FullTrust" permission.

If you want to set permission to specific assembly here is a guide from Microsoft Docs:

"You can associate your own custom levels of permissions for custom assemblies that you use with reports. For example, if you want to allow an assembly to access a specific file, you can create a new named permission set with specific file I/O access and then assign the permission set to your code group.

<CodeGroup class="UnionCodeGroup"  
   version="1"  
   PermissionSetName="FullTrust"  
   Name="MyNewCodeGroup"  
   Description="A special code group for my custom assembly.">  
   <IMembershipCondition class="UrlMembershipCondition"  
      version="1"  
      Url="C:\Program Files\Microsoft Power BI Report Server\PBIRS\ReportServer\bin\MyCustomAssembly.dll"/>  
</CodeGroup>

"