You cannot add or change a record because a related record is required in table

private void FunInsOrderDetails()
        {
            using (OleDbConnection connection = new OleDbConnection(DataConnection.constring))
            {
                OleDbTransaction transaction = null;
                try
                {
                    OleDbCommand command = connection.CreateCommand();                  
                    connection.Open();
                    transaction = connection.BeginTransaction(); // Assign Transaction to Command
                    command.Transaction = transaction;
                   command.CommandText= "insert into Custorder(OrderType,OrderNo,OrderDate,YourOrderNo,CustSuppId)values('" + cmbOrderType.Text + "'," + txtOrderNo.Text + ",'" + dtpDate.Value.ToString("yyyy-MM-dd") + "','" + txtYourOrderNo.Text + "'," + cmbCustName.SelectedValue.ToString() + ")";                  
                   command.ExecuteNonQuery();                   
                    FunInsOrder(command);
                    transaction.Commit();
                    connection.Close();
                }
                catch
                {
                    transaction.Rollback();
                    connection.Close();
                    throw;
                }
            }           
        }

private void FunInsOrder(OleDbCommand cmd)
      {
          for (int i = 0; i < dgvOrder.Rows.Count - 1; i++)
          {
              str = "select p.ProjectId from Project p INNER JOIN Machine m on m.MachineId=p.MachineId where p.DrawingNo='"+dgvOrder.Rows[i].Cells[1].Value+"' AND p.PartName='"+dgvOrder.Rows[i].Cells[2].Value+"'AND m.MachineName='"+dgvOrder.Rows[i].Cells[0].Value+"'";
              int ProjectId = dc.RetutnID(str);
              str="Select OrderRepairId from CustOrder where OrderNo="+txtOrderNo.Text+" AND OrderType='"+cmbOrderType.Text+"'";
              int OrderRepairId = dc.RetutnID(str);
              if (cmbOrderNo.Visible == false)
              {
                  cmd.CommandText= "insert into OrderDetails(OrderType,OrderRepairId,OrderDate,ProjectId,MachineName,Quantity)values('"+cmbOrderType.Text+"'," + OrderRepairId.ToString()+ ",'" + dtpDate.Value.ToString("yyyy-MM-dd") + "'," + ProjectId.ToString() + ",'" + dgvOrder.Rows[i].Cells[3].Value + "'," + dgvOrder.Rows[i].Cells[4].Value + ")";
              }
              else
              {
                  cmd.CommandText = "insert into OrderDetails(OrderType,OrderRepairId,OrderDate,ProjectId,MachineName,Quantity)values('" + cmbOrderType.Text + "'," + cmbOrderNo.SelectedValue.ToString() + ",'" + dtpDate.Value.ToString("yyyy-MM-dd") + "'," + ProjectId.ToString() + ",'" + dgvOrder.Rows[i].Cells[3].Value + "'," + dgvOrder.Rows[i].Cells[4].Value + ")";
              }
              cmd.ExecuteNonQuery();
          }
      }