Keeps getting error: AutErcNoDoc 1 No document is currently open in the Acrobat application.

Hi I am completely new to Acrobat SDK.  I am trying to fill PDF form from An Excel Macro.  Windows 7, Excel 2013 and Acrobat XI Pro are currently used.

I have added the AFormAut and Acrobat type library in preferences. 

My code included these

    Set AcrobatApplication = CreateObject("AcroExch.App")

    Set AcrobatDocument = CreateObject("AcroExch.AVDoc")

    If AcrobatDocument.Open("....", "") Then

        Set AcroForm = CreateObject("AFormAut.App")

        Set Fields = AcroForm.Fields ' ERROR HAPPENS

     End If

The error was always reported at the AcroForm.Fields line.

I have tried all kinds of AcrobatApplication.Show  Hide AcrobatDocument.Maximize(1)  BringToFront  as suggested by some old threads but none worked.  I can really use some help!  Thanks ahead!