Were you able to resolve the issue singhal?
I am having the same problem, using either one similar to the token in the sample and one with an exipration set as per Eric's.
IEnumerable<string> UserRoles = new List<string>() {"provider"};
var embedToken = PowerBIToken.CreateReportEmbedToken(this.workspaceCollection, this.workspaceId, report.Id, "10005", UserRoles);
var embedToken = PowerBIToken.CreateReportEmbedToken(this.workspaceCollection, this.workspaceId, report.Id, Convert.ToDateTime("2017-12-12"), "10005", new string[] { "provider" });