The permissions granted to user ' are insufficient for performing this operation. (rsAccessDenied)"}

under Site setting in Reports manager >Configure system-level role definitions > check ExecuteReport Defination option then Create a System UserGroup, Give the access to that group at Connect to your reporting Services Data base in server properties and add a group and permite the access as System User... It should work

I know it's for a long time ago but may be helpful to any other new comers,

I decided to pass user name,password and domain while requesting SSRS reports, so I created one class which implements IReportServerCredentials.

 public class ReportServerCredentials : IReportServerCredentials   
{
    #region  Class Members
        private string username;
        private string password;
        private string domain;
    #endregion

    #region Constructor
        public ReportServerCredentials()
        {}
        public ReportServerCredentials(string username)
        {
            this.Username = username;
        }
        public ReportServerCredentials(string username, string password)
        {
            this.Username = username;
            this.Password = password;
        }
        public ReportServerCredentials(string username, string password, string domain)
        {
            this.Username = username;
            this.Password = password;
            this.Domain = domain;
        }
    #endregion

    #region Properties
        public string Username
        {
            get { return this.username; }
            set { this.username = value; }
        }
        public string Password
        {
            get { return this.password; }
            set { this.password = value; }
        }
        public string Domain
        {
            get { return this.domain; }
            set { this.domain = value; }
        }
        public WindowsIdentity ImpersonationUser
        {
            get { return null; }
        }
        public ICredentials NetworkCredentials
        {
            get
            {
                return new NetworkCredential(Username, Password, Domain);
            }
        }
    #endregion

    bool IReportServerCredentials.GetFormsCredentials(out System.Net.Cookie authCookie, out string userName, out string password, out string authority)
    {
        authCookie = null;
        userName = password = authority = null;
        return false;
    }
}

while calling SSRS Reprots, put following piece of code

 ReportViewer rptViewer = new ReportViewer();
 string RptUserName = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportUser"]);
        string RptUserPassword = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportUserPassword"]);
        string RptUserDomain = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportUserDomain"]);
        string SSRSReportURL = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportURL"]);
        string SSRSReportFolder = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportFolder"]);

        IReportServerCredentials reportCredentials = new ReportServerCredentials(RptUserName, RptUserPassword, RptUserDomain);
        rptViewer.ServerReport.ReportServerCredentials = reportCredentials;
        rptViewer.ServerReport.ReportServerUrl = new Uri(SSRSReportURL);

SSRSReportUser,SSRSReportUserPassword,SSRSReportUserDomain,SSRSReportFolder are defined in web.config files.

The report might want to access a DataSource or DataView where the AD user (or AD group) has insuficcient access rights.

Make sure you check out the following URLs:

• http://REPORTSERVERNAME/Reports/Pages/Folder.aspx?ItemPath=%2fDataSources
• http://REPORTSERVERNAME/Reports/Pages/Folder.aspx?ItemPath=%2fDataSets

Then choose Folder Settings

(or the appropriate individual DataSource or DataSet) and select Security. The user group needs to have the Browser permission.