I can't connect to my SQL Server database Using VB.NET

Protected Sub login_btn_Click(sender As Object, e As EventArgs) Handles login_btn.Click

    Dim connString As String
    Dim connection As SqlConnection
    Dim command As SqlCommand
    Dim sql As String
    connString = "Data Source=.\SQLExpress;Initial Catalog=Suivi_Invst;Integrated Security=True"
    sql = "select NET_ID, Password from User"
    connection = New SqlConnection(connString)

    Try
        connection.Open()
        command = New SqlCommand(sql, connection)
        Dim sqlReader As SqlDataReader = command.ExecuteReader()
        While sqlReader.Read()
            If (Password.Text = sqlReader("Password") And NET_ID.Text = sqlReader("NET_ID")) Then
                Response.Redirect("Creation.aspx")
            End If
        End While

        sqlReader.Close()
        command.Dispose()
        connection.Close()
    Catch ex As Exception
        MsgBox("Can not open connection ! ")
    End Try

End Sub

I always get the msg box saying that connection can noy be established.

I tried to open SQL Server Management Studio at the same time the code running but I didn't anything.