How do i show popup error messsage when textbox is empty

I want to use this code below to show user that the recipt textbox is empty but the button OnSave is showing error

see link for the complete code

Insert (Save) sum of unit price and quantity using UserControl in ASP.Net

<div class="form-group-inner">
    <div class="row">
        <div class="">
            <div class="input-group mg-b-pro-edt">
                <span class="input-group-addon"><i class="fa fa-sticky-note-o" aria-hidden="true"></i></span>
                <asp:TextBox ID="txtrecipt" runat="server" CssClass="form-control" placeholder="Recipt" ReadOnly="True"></asp:TextBox>
            </div>
        </div>
    </div>
</div>

protected void OnSave(object sender, EventArgs e)
{
    if (Page.IsValid)
    {
        if (txtrecipt.Text == "")
        {
            ScriptManager.RegisterClientScriptBlock(OnSave, this.GetType(), "alert", "<script>alert('Enter Quantity Ordered ... !!')</script>", false);
        }
        else
        {
            // lblMessage2.Text = "";
            DataTable dt = new DataTable();
            dt.Columns.AddRange(new DataColumn[] { new DataColumn("Item"), new DataColumn("Price"), new DataColumn("Quantity") });
            foreach (Control c in ph1.Controls)
            {
                if (c.GetType().Name.ToLower() == "usercontrol_ascx")
                {
                    UserControl uc = (UserControl)c;

                    TextBox tbItem = uc.FindControl("txtItem") as TextBox;
                    TextBox tbPrice = uc.FindControl("txtPrice") as TextBox;
                    TextBox tqty = uc.FindControl("txtQuantity") as TextBox;
                    if (!string.IsNullOrEmpty(tbItem.Text.Trim()) && !string.IsNullOrEmpty(tbPrice.Text.Trim()) && !string.IsNullOrEmpty(tqty.Text.Trim()))
                    {
                        dt.Rows.Add(tbItem.Text.Trim(), tbPrice.Text.Trim(), tqty.Text.Trim());
                        if (Convert.ToInt32(tqty.Text.Trim()) > 0)
                        {
                            int availableQuantity = Convert.ToInt32(GetAvailableQuantity(tbItem.Text.Trim()));
                            if (Convert.ToInt32(tqty.Text.Trim()) > availableQuantity)
                            {
                                ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('No of Quantity entered is not available in Stock')", true);
                            }
                            else
                            {
                                int inserted = Insert(tbItem.Text.Trim(), tbPrice.Text.Trim(), tqty.Text.Trim());
                                if (inserted > 0)
                                {
                                    UpdateStock(tbItem.Text.Trim(), tqty.Text.Trim());
                                    ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('Data Submitted Successfully ... !!')", true);
                                }
                            }
                        }
                    }
                }
            }
            // BindGridID();
        }
    }
}