Windows Form Error: The INSERT statement conflicted with the FOREIGN KEY constraint

below my code i have two table salesorderAddeidt its is a procedure apply on saleorder table to get value from textboxes,and second table is orderdetail storeprocedure is OrderDetailAddorEdit to get value from datagridvew

In window form,error Additional information: The INSERT statement conflicted with the FOREIGN KEY constraint "FK_OrderDetail_SalesOrder". The conflict occurred in database "SilverProduction", table "dbo.SalesOrder", column 'OrderNo'

       private void btnSave_Click(object sender, EventArgs e)
       {
           if (ValidateMasterDetailForm())
           {
               int _EmpID = 0;
               using (SqlConnection sqlCon = new SqlConnection(strConnectionString))
               {
                   sqlCon.Open();
                   //Master[SalesOrderAddOrEdit]
                   SqlCommand sqlCmd = new SqlCommand("SalesOrderAddOrEdit", sqlCon);
                   sqlCmd.CommandType = CommandType.StoredProcedure;
                    sqlCmd.Parameters.AddWithValue("@OrderNo", inEmpID);
                  // sqlCmd.Parameters.AddWithValue("@UserId", CType(Me.Session("UserDetails"), User).UserId);
                   sqlCmd.Parameters.AddWithValue("@Order_Ref_No", txtOrderRefNo.Text.Trim());
                   sqlCmd.Parameters.AddWithValue("@CustomerID", Convert.ToInt32(cmbCustomerName.SelectedValue.ToString()));
                   sqlCmd.Parameters.AddWithValue("@Order_date", dateTimePicker1.Value);
                   sqlCmd.Parameters.AddWithValue("@SOpirority", txtSO.Text.Trim());
                   sqlCmd.Parameters.AddWithValue("@Status", cmbStatus.Text);
 
                   _EmpID = Convert.ToInt32(sqlCmd.ExecuteScalar());
               }
               //Details
               using (SqlConnection sqlCon = new SqlConnection(strConnectionString))
               {
                   sqlCon.Open();
                   foreach (DataGridViewRow dgvRow in dgvOrderDetail.Rows)
                   {
                       if (dgvRow.IsNewRow) break;
                       else
                       {
                           SqlCommand sqlCmd = new SqlCommand("OrderDetailAddOrEdit", sqlCon);
                           sqlCmd.CommandType = CommandType.StoredProcedure;
                           sqlCmd.Parameters.AddWithValue("@IDOD", Convert.ToInt32(dgvRow.Cells["dgvtxtIDOD"].Value == DBNull.Value ? "0" : dgvRow.Cells["dgvtxtIDOD"].Value));
                           sqlCmd.Parameters.AddWithValue("@OrderNo", _EmpID);
                           sqlCmd.Parameters.AddWithValue("@CodeItem", dgvRow.Cells["dgvtxtCodeItem"].Value == DBNull.Value ? "" : dgvRow.Cells["dgvtxtCodeItem"].Value);
                           sqlCmd.Parameters.AddWithValue("@orderqty", dgvRow.Cells["dgvorderqty"].Value == DBNull.Value ? "" : dgvRow.Cells["dgvorderqty"].Value);
 
                           sqlCmd.ExecuteNonQuery();
                       }
                   }
               }
               FillEmployeeDataGridView();
               Clear();
               MessageBox.Show("Submitted Successfully");
           }
       }

USE [SilverProduction]
GO
/***** Object:  StoredProcedure [dbo].[SalesOrderAddOrEdit]    Script Date: 06/27/2019 02:20:05 *****/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER Proc [dbo].[SalesOrderAddOrEdit]
@OrderNo int,
@Order_Ref_No varchar(50),
@CustomerID int,
@Order_date date,
@SOpirority int,
@Status varchar(50)
AS
 
--Insert
 IF @OrderNo=0 Begin
    insert into SalesOrder(Order_Ref_No,CustomerID,Order_date,SOpirority, Status) VALUES( @Order_Ref_No,@CustomerID,@Order_date,@SOpirority, @Status)
 end
 --Update
 Else Begin
  Update SalesOrder
  set
  
  Order_Ref_No=@OrderNo,
  CustomerID=@CustomerID,
  Order_date=@Order_date,
  SOpirority=@SOpirority,
  Status=@Status 
   
 where OrderNo=@OrderNo
  
 select @OrderNo;
 End
 

USE [SilverProduction]
GO
/***** Object:  StoredProcedure [dbo].[OrderDetailAddOrEdit]    Script Date: 06/27/2019 02:16:43 *****/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER Proc [dbo].[OrderDetailAddOrEdit]
@IDOD int,
@OrderNo int,
@CodeItem int,
@orderqty int
 
As
 --Insert
    IF @OrderNo = 0
    insert into OrderDetail(OrderNo,CodeItem,orderqty) VALUES( @OrderNo,@CodeItem,@orderqty)
 --UPDATE
 ELSE
 UPDATE OrderDetail
 SET
   OrderNo=@OrderNo,
   CodeItem=@CodeItem,
   orderqty=@orderqty
 
 WHERE IDOD = @IDOD