The permissions granted to user '' are insufficient for performing this operation. (rsAccessDenied)
You are working on some operations say reporting building or may be a folder creation in SSRS Report Manager and when you say ok you are hit with the below error.
"The permissions granted to user '' are insufficient for performing this operation. (rsAccessDenied)"
Probable reason as the error itself says is you are not having permissions to say create folder as only browser permissions are granted to you.
To resolve this, you can ask your IT Team to provide you with required permission.
Now, say that you are local administrator on this Machine where SSRS is installed, but still getting this error, well no worries. You can have this resolved yourself.
Follow the procedures provided in the below Link, in which I have explained how to get access to SSRS Report Manager where you are local administrator.