Username already exist

Please help me, my code won't work :(
I want it to check if username already exists

I'm using VS2013 and MS Access 2013

If btnSave.Text = "Save" Then
                Dim ct1 As String = "SELECT [Username] FROM tblCredentials WHERE Username='" & txtUsername.Text & "'"
                cmd = New OleDb.OleDbCommand(ct1)
                cmd.Connection = cnn
                rdr = cmd.ExecuteReader()

                If rdr.Read Then
                    MessageBox.Show("Username already exists!", "Error", MessageBoxButtons.OK, MessageBoxIcon.[Error])
                    txtUsername.SelectAll()
                    txtUsername.Focus()
                    If Not rdr Is Nothing Then
                        rdr.Close()
                    End If
                    Exit Sub
                End If
            End If


            If Me.txtMID.Tag & "" = "" Then

                'add data to table
                cmd.CommandText = "INSERT INTO [tblCredentials]([MemberID], [FirstName], [MiddleName], [LastName], [Address], [Username], [Password], [ConfirmPass], [Position]) " & _" VALUES(" & Trim(Me.txtMID.Text) & ",'" & Trim(Me.txtFirst.Text) & "','" & Trim(Me.txtMiddle.Text) & "','" & Trim(Me.txtLast.Text) & "','" & Trim(Me.txtAddress.Text) & "','" & Trim(Me.txtUsername.Text) & "','" & _
                                Trim(Me.txtPass.Text) & "','" & Me.txtConfirmPass.Text & "','" & _
                                Me.cboPosition.Text & "')"
                cmd.ExecuteNonQuery()

            Else
                cmd.CommandText = "UPDATE [tblCredentials] " & _" SET [MemberID]=" & Me.txtMID.Text & _", [FirstName]='" & Trim(Me.txtFirst.Text) & "'" & _", [MiddleName]='" & Trim(Me.txtMiddle.Text) & "'" & _", [LastName]='" & Trim(Me.txtLast.Text) & "'" & _", [Address]='" & Trim(Me.txtAddress.Text) & "'" & _", [Username]='" & Trim(Me.txtUsername.Text) & "'" & _", [Password]='" & Trim(Me.txtPass.Text) & "'" & _", [ConfirmPass]='" & Me.txtConfirmPass.Text.Trim & "'" & _", [Position]='" & Me.cboPosition.Text & "'" & _" WHERE [MemberID]=" & Me.txtMID.Tag
                cmd.ExecuteNonQuery()