Thanks @georgiboy - So I created 2 subfolders within shared mailbox but outside the shared mailbox inbox and run the code by changing below 3 lines. Unfortunately it's still giving me the same error message. The code works fine on the personal email account. Do you think it's something to do with the permissions on the shared mailbox? 

 Set ShareInbox = mynamespace.Session.Folders.Item(ProdMail) ' change to suit
    Set SubFolder = ShareInbox.Folders.Item(InputFolder) 'Change this line to specify folder
    Set myDestFolder = ShareInbox.Folders.Item(OutputFolder)