"The ConnectionString property has not been initialized." Error in SQL bulkcopy upload

I just developed an MVC 4 application with the ability to upload Excel files to databases.

this is successfully operating on localhost.

But when I attempt to submit an excel file when deployed in IIS. I'm encountering the next error

The ConnectionString property has not been initialized.

To upload excel files, I'm using these connection strings.

<add name="dbconnection" connectionString="Data Source="000.000.00.00";Initial Catalog=AFFHEC_DB;Persist Security Info=True;User ID=**;Password=****" providerName="System.Data.SqlClient" />
<add name="constr" connectionString="data source=000.000.0.000;Initial catalog=AFFHEC_DB;user id=**;password=*****;" />
<add name="Excel07+ConString" connectionString="Provider=Microsoft.ACE.OLEDB.12.0;Data Source={0};Extended Properties='Excel 8.0;HDR=YES';" />
<add name="Excel03ConString" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source={0};Extended Properties='Excel 8.0;HDR=YES'" />

This is the SQL bulk copy upload code.

    [HttpPost]
    [AcceptVerbs(HttpVerbs.Post)]
    public ActionResult Student(HttpPostedFileBase FileUpload1, tbl_hec_Programme programme)
    {

        try
        {
            string conString = string.Empty;
            //Upload and save the file


           if (Request.Files["FileUpload1"].ContentLength > 1)
            {
                try
                {
                    string excelPath = Path.Combine(HttpContext.Server.MapPath("~/Content/"), Path.GetFileName(FileUpload1.FileName));
                    FileUpload1.SaveAs(excelPath);


                    string extension = System.IO.Path.GetExtension(FileUpload1.FileName);
                                            conString = string.Format(conString, excelPath);  
                }
                catch (Exception ex)
                {

                    this.SetNotification("The file type submitted is invalid", NotificationEnumeration.Error);

                }

                try
                {

                   using (OleDbConnection excel_con = new OleDbConnection(conString))
                    {
                        excel_con.Open();

                        DataTable dtExcelData = new DataTable();

                        string query =    "SELECT " +
                                          "s1.HEC_ID, " +
                                          "s1.Student_Personal_ID, " +
                                          "s1.Student_Passport_Number, " +
                                          "s1.Student_ID_by_University, " +
                                          "s1.First_Name, " +


                         //    // join three tables
                        "FROM (((([Personal_Data$] as s1) " +
                        "LEFT OUTER JOIN [Enrolment_Data$] as s2 ON s1.HEC_ID = s2.HEC_ID) " +

                        using (OleDbDataAdapter oda = new OleDbDataAdapter(query, excel_con))
                        {
                            oda.Fill(dtExcelData);
                        }

                        string consString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
                        using (SqlConnection con = new SqlConnection(consString))
                        {
                            using (SqlBulkCopy sqlBulkCopy = new SqlBulkCopy(con,
                                                        SqlBulkCopyOptions.CheckConstraints |
                                                        SqlBulkCopyOptions.FireTriggers |
                                                        SqlBulkCopyOptions.KeepNulls |
                                                        SqlBulkCopyOptions.TableLock |
                                                        SqlBulkCopyOptions.UseInternalTransaction |
                                                        SqlBulkCopyOptions.KeepIdentity,
                                                        null))
                            {
                                //Set the database table name
                                sqlBulkCopy.DestinationTableName = "tbl_HEI_student";

                                sqlBulkCopy.BulkCopyTimeout = 0;

                                sqlBulkCopy.ColumnMappings.Add("ID", "ID");
                                sqlBulkCopy.ColumnMappings.Add("Student_Personal_ID", "Student_Personal_ID_CPR");
                                sqlBulkCopy.ColumnMappings.Add("Student_Passport_Number", "Student_Passport_Number");
                                ................


                                con.Open();

                                try
                                {

                                    sqlBulkCopy.WriteToServer(dtExcelData);

                                    int totalRowsAdded = dtExcelData.Rows.Count;


                                    UploadStatusWriter(User.Identity.GetUserId(), Status_type, Date_type);



                                    this.SetNotification("Student data successfully uploaded", NotificationEnumeration.Success);



                                    return RedirectToAction("StudentIndex", "HEI");





                                }
                                catch (Exception ex)
                                {
                                    this.SetNotification(ex.Message, NotificationEnumeration.Error);
                                    return RedirectToAction("Student", "Excel");
                                }
                                finally
                                {

                                    con.Close();


                                }
                            }
                        }
                    }
                }
                catch (Exception ex)
                {
                    this.SetNotification(ex.Message, NotificationEnumeration.Error);
                    return RedirectToAction("Student", "Excel");
                }

            }

            else
            {
                this.SetNotification("Please select a file first and then click the submit button", NotificationEnumeration.Error);
                return RedirectToAction("Student", "Excel");

            }



        }
        catch (Exception ex)
        {
            return RedirectToAction("Student", "Excel");
        }

    }