Error To Come Up On A Label Stating "please Enter Remarks" When Text Field Is Empty

If a user selects "No" from BicAdjddl and the Notxt is empty I don't want the page to submit I want an error to come up on a label stating please enter remarks. Here's the dropdown and text box.

<asp:DropDownList ID="BicAdjddl" runat="server">
<asp:ListItem>Yes</asp:ListItem>
<asp:ListItem>No</asp:ListItem>
<asp:ListItem Selected="True" Value="0">Select Yes or No</asp:ListItem>
</asp:DropDownList></td>
<asp:TextBox
ID="Notxt"
runat="server"
TextMode="MultiLine"></asp:TextBox></td>

Here's my button click...what I added isn't working I added it in the try catch block:

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim conn As New Data.SqlClient.SqlConnection(ConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString)
Dim cmd As New Data.SqlClient.SqlCommand
With cmd
.Connection = conn 'the connection
.CommandType = Data.CommandType.StoredProcedure
.CommandText = "UpdateClaim"
.Parameters.AddWithValue("@claim", Request.QueryString("claim"))
.Parameters.AddWithValue("@pic", Request.QueryString("pic"))
.Parameters.AddWithValue("@Remarks", Remarkstxt.Text)
.Parameters.AddWithValue("@BicAdj", BicAdjddl.SelectedValue)
.Parameters.AddWithValue("@NoBIC", NOTxt.Text)
End With
Try
If BicAdjddl.SelectedValue = "No" Then
BIClbl.Text = "enter some remarks"
End If
Catch ex As Exception
End Try
conn.Open()
cmd.ExecuteNonQuery()
End Sub