You are calling the Windows version of the app. You need to use the Server for line commands. Change your line of code to this:

  PDFtkPath = "C:\Program Files (x86)\PDFtk Server\bin\pdftk.exe"
You still need to use my version of the shell call with the quotes if you have any spaces in the file path. I would recommend you do it anyway.

    ShellCmd = PDFtkPath & " """ & InputFilePath & """ output """ & OutputFilePath & """ user_pw " & Password