It means you are trying to access the files from a folder which is not present.
Check the values from the lstfolder list and verify physically that the folders are present or not.
in code:
DirectoryInfo dir = new DirectoryInfo(lstfolder.SelectedValue);
if (dir.Exists)
{
foreach (FileInfo fi in dir.GetFiles())
{
lstfile.Items.Add(fi.ToString());
}
}
refer:
DirectoryInfo.Exists [
^]