How to Get RootFolderId By UserId in DescribeUser Amazon Workdocs API?

I have UserId, and I need the root folder id for that particular user. What I have done is, used the DescribeUser API of AmazonWorkdocs And provided id in UserIds Query. This API is working fine in postman. But when I'm using the same for My project (NetCore) it returns {{"Message": "Invalid SigV4 credentials."}} The code is provided below.

var userRequest = new ESBMessage();
                    userRequest.LocalPath = String.Format("/api/v1/users");
                    userRequest.Http.Method = "GET";
                    //userRequest.Http.Query.Add("organizationId", OrganizationId);
                    userRequest.Http.Query.Add("userIds", userId);// UserId contains id of user
                    SetAuthenticationHeader(userRequest);
                    var driveList = _client.CallRestApi(userRequest, GetRequestURI(), true);