Implicit conversion from data type nvarchar to varbinary(max) is not allowed. Use the CONVERT function to run this query.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.IO;
using System.Web.UI.WebControls;

public partial class Medicaldetails : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["hospitalConnectionString"].ToString());
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            BindEmployeeDetails();
        }



    }

    protected void BindEmployeeDetails()
    {
        con.Open();
        SqlCommand cmd = new SqlCommand("Select * from medicine", con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();

        da.Fill(ds);
        con.Close();
        if (ds.Tables[0].Rows.Count > 0)
        {
            gvDetails.DataSource = ds;

            gvDetails.DataBind();
        }
        else
        {
            ds.Tables[0].Rows.Add(ds.Tables[0].NewRow());
            gvDetails.DataSource = ds;
            gvDetails.DataBind();
            con.Close();
            int columncount = gvDetails.Rows[0].Cells.Count;
            gvDetails.Rows[0].Cells.Clear();
            gvDetails.Rows[0].Cells.Add(new TableCell());
            gvDetails.Rows[0].Cells[0].ColumnSpan = columncount;
            gvDetails.Rows[0].Cells[0].Text = "No Records Found";
        }
    }

    protected void gvDetails_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
    {
        gvDetails.EditIndex = -1;
        BindEmployeeDetails();

    }
    protected void gvDetails_RowEditing(object sender, GridViewEditEventArgs e)
    {
        gvDetails.EditIndex = e.NewEditIndex;
        BindEmployeeDetails();

    }
    protected void gvDetails_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
        int s_no = Convert.ToInt32(gvDetails.DataKeys[e.RowIndex].Values["s_no"].ToString());
        TextBox bill_number = (TextBox)gvDetails.Rows[e.RowIndex].FindControl("bill_number");
        TextBox date = (TextBox)gvDetails.Rows[e.RowIndex].FindControl("txt_date");
        TextBox shop_name = (TextBox)gvDetails.Rows[e.RowIndex].FindControl("txt_shopname");
        TextBox ammount = (TextBox)gvDetails.Rows[e.RowIndex].FindControl("txt_amt");
        con.Open();
        SqlCommand cmd = new SqlCommand("update medicine set  bill_number='" + bill_number.Text + "', date = '" + date.Text + "', shop_name ='" + shop_name.Text + "', ammount = '" + ammount.Text + "'  where  s_no=" + s_no, con);
        cmd.ExecuteNonQuery();
        con.Close();
        gvDetails.EditIndex = -1;
        BindEmployeeDetails();

    }
    protected void gvDetails_RowDeleting(object sender, GridViewDeleteEventArgs e)
    {
        int s_no = Convert.ToInt32(gvDetails.DataKeys[e.RowIndex].Values["s_no"].ToString());
        con.Open();
        SqlCommand cmd = new SqlCommand("delete from medicine where  s_no  =" + s_no, con);
        int result = cmd.ExecuteNonQuery();
        con.Close();
        if (result == 1)
        {
            BindEmployeeDetails();


        }
    }
    protected void gvDetails_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName.Equals("AddNew"))
        {
            TextBox bill_number = (TextBox)gvDetails.FooterRow.FindControl("txt_bill");
            TextBox date = (TextBox)gvDetails.FooterRow.FindControl("txtDate");
            TextBox shop_name = (TextBox)gvDetails.FooterRow.FindControl("txt_shopname11");
            TextBox Ammount = (TextBox)gvDetails.FooterRow.FindControl("txt_amt11");
            FileUpload fileupd2 = (FileUpload)gvDetails.FooterRow.FindControl("FileUpload1_test");
            
            FileUpload fileupd1 = (FileUpload)gvDetails.FooterRow.FindControl("FileUpload_bill");
            FileUpload fileupd = (FileUpload)gvDetails.FooterRow.FindControl("FileUpload_pre");
            con.Open();

            SqlCommand cmd = new SqlCommand("insert into medicine(bill_number, dt, shop_name, Ammount,pres, medicine_bills, test_bills ) values(@bill_number, @dt, @shop_name, @Ammount, @pres, @medicine_bills, @test_bills)", con);
            cmd.Connection = con;
            cmd.Parameters.AddWithValue("@bill_number", bill_number.Text);
            cmd.Parameters.AddWithValue("@dt", date.Text);
            cmd.Parameters.AddWithValue("@shop_name", shop_name.Text);
            cmd.Parameters.AddWithValue("@Ammount", Ammount.Text);
            cmd.Parameters.AddWithValue("@pres", fileupd.FileName);
            cmd.Parameters.AddWithValue("@medicine_bills", fileupd1.FileName);
            cmd.Parameters.AddWithValue("@test_bills", fileupd2.FileName);


            int result = cmd.ExecuteNonQuery();
            con.Close();
            if (result == 1)
            {
                BindEmployeeDetails();
            }

            else
            {
              
            }
        }
    }
    protected void gvDetails_SelectedIndexChanged(object sender, EventArgs e)
    {
        FileUpload fileupd2 = (FileUpload)gvDetails.FooterRow.FindControl("FileUpload1_test");
        FileUpload fileupd1 = (FileUpload)gvDetails.FooterRow.FindControl("FileUpload_bill");
        FileUpload fileupd = (FileUpload)gvDetails.FooterRow.FindControl("FileUpload_pre");
        string filename = Path.GetFileName(fileupd.PostedFile.FileName);
        string filename_bill = Path.GetFileName(fileupd1.PostedFile.FileName);
        string filename_test = Path.GetFileName(fileupd2.PostedFile.FileName);
        Stream str = fileupd.PostedFile.InputStream;
        Stream str1 = fileupd1.PostedFile.InputStream;
        Stream str2 = fileupd2.PostedFile.InputStream;
        BinaryReader br = new BinaryReader(str);
        Byte[] size = br.ReadBytes((int)str.Length);
      
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = con;
        cmd.CommandText = " insert into medicine (pres, medicine_bills, test_bills) values(@pres, @medicine_bills, @test_bills)";
       
        cmd.Parameters.AddWithValue("@pres", fileupd.FileName);
        cmd.Parameters.AddWithValue("@medicine_bills", fileupd1.FileName);
        cmd.Parameters.AddWithValue("@test_bills", fileupd2.FileName);
        con.Open();
        cmd.ExecuteNonQuery();
        con.Close();
        BindEmployeeDetails();





            
    }
}

CREATE TABLE [dbo].[medicine](
    [s_no] [int] IDENTITY(1,1) NOT NULL,
    [bill_number] [int] NULL,
    [dt] [date] NULL,
    [shop_name] [varchar](88) NULL,
    [Ammount] [varchar](88) NULL,
    [medicine_bills] [varbinary](max) NULL,
    [test_bills] [varbinary](max) NULL,
    [pres] [varbinary](max) NULL,