Unable to open the file error when trying to open .txt file

The problem is in your declaration of sPath.  Because you're using JavaScript, the backslash (\) character is a control character.  In order to have that character in a string, you need to double it.  Change your code as noted below paying special attention to the bolded line.

function RetrieveExistingPaitent()
  {
  var sPath = "C:\\Files\\MyFile.txt";

  // Opens the specified file for reading
  var myFile = aqFile.OpenTextFile(sPath, aqFile.faRead, aqFile.ctUnicode);
  Log.Message("File by lines:");

  // Reads text lines from the file and posts them to the test log
  while(! myFile.IsEndOfFile())
  {
    s = myFile.ReadLine();
    Log.Message(s);
  }

  // Closes the file
  myFile.Close();
}