how to get files from sub-folders using "GetFilesAsync()" method

You're actually not fetching the files from the selected folder itself.

Add this after "StorageFolder folder = await op.PickSingleFolderAsync();":

IReadOnlyList<StorageFile> rootFiles = await folder.GetFilesAsync();

foreach (StorageFile file in rootFiles)
{
    TextBlock1.Text += file.DisplayName.ToString() + Environment.NewLine;
}