Redirect to error page when entered code be wrong in ASP.Net

Try This

protected void Trade_Click(object sender, ImageClickEventArgs e)
   {
  
       string data = Server.UrlEncode(txtNumeric.Text);
       SqlCommand _cmd = new SqlCommand("traidname", _cn);
       _cmd.CommandType = CommandType.StoredProcedure;
       _cmd.Parameters.Add("@BehCode", data);
       _cn.Open();
        object tName=_cmd.ExecuteScalar();
        _cn.Close();
        if(tName !=null && tName!= DBNull.Value)
        {
            Response.Redirect(tName + "?BehCode=" + Server.UrlEncode(txtNumeric.Text));
        }
        else
        {
            Response.Redirect("~/Error.aspx");
        }     
}

After assing value in data variable

string data = Server.UrlEncode(txtNumeric.Text);

when you pass data varialble value as parameter value to @Behcode if procedure traidname retun any output it will not through any error but if traidname procedure not match @behcode value then no row will affected.

_cmd.ExecuteScalar().ToString(); 

will not return any value it will cause an error

so replace it as

_cmd.ExecuteScalar();