【解決方法】パラメータ化されたクエリ ‘(@Account_No nvarchar(4000),@Account_title nvarchar(4000),@Entry’ はパラメータ ‘@Account_type’ を予期していますが、これは指定されていません。

private void btnsave_Click(object sender, EventArgs e)
        {
            String query = "insert into Accounts(Account_No,Account_title,Entry_Date,Initial_Deposite,Account_type,Account_Nature,Postal_Code,District,City,Name,Father_name,Husband_name)values(@Account_No,@Account_title,@Entry_Date,@Initial_Deposite,@Account_type,@Account_Nature,@Postal_Code,@District,@city,@Name,@Father_name,@Husband_name,@Occupation,@Mobile,@CNIC)";
            SqlCommand cmd = new SqlCommand(query, cn);
            cmd.Prepare();
            cmd.Parameters.AddWithValue("@Account_No", txtaccount1.Text);
            //cmd.Parameters.Add("@Account_No", System.Data.SqlDbType.NVarChar).Value = txtaccount1.Text;
            cmd.Parameters.AddWithValue("@Account_title", txtaccounttitle.Text);
           // cmd.Parameters.Add("@Account_title", System.Data.SqlDbType.NVarChar).Value = txtaccounttitle.Text;
            cmd.Parameters.AddWithValue("@Entry_Date", dateTimePicker1.Value.Date);
            cmd.Parameters.AddWithValue("@Initial_Deposite", txtinitial.Text);
            //cmd.Parameters.AddWithValue("@Account_type", txtaccounttype.Text);
            cmd.Parameters.AddWithValue("@Account_Nature", txtaccountnature.Text);
            cmd.Parameters.AddWithValue("@Account_type",cboaccounttype.SelectedValue);
          
            cmd.Parameters.AddWithValue("@Postal_code", txtpostal.Text);
            cmd.Parameters.AddWithValue("@City", cbocity.SelectedValue);
            cmd.Parameters.AddWithValue("@District", cbodistrict.SelectedValue);
                
            cmd.Parameters.AddWithValue("@Name", txtname1.Text);
            cmd.Parameters.AddWithValue("@Father_name", txtfather1.Text);
            cmd.Parameters.AddWithValue("@Husband_name", txthusband1.Text);
            cmd.Parameters.AddWithValue("@Occupation", txtoccupation1.Text);
            cmd.Parameters.AddWithValue("@Mobile", txtmobile1.Text);
            cmd.Parameters.AddWithValue("@CNIC", txtcnic1.Text);
            cmd.ExecuteNonQuery();
            MessageBox.Show("record stored");
                   
        }
            }
        }