Go to View>Folders. You now need to tap the folder icon next to Inbox and it will 'expand' and display all the subfolders under it. This MUST be the icon and not the text. It wont work if you press the text so you must press the icon. I have to say this is pretty poor UI design, as it is not obvious to end users.
Various Symptoms which cannot synchronize subscribed folders problem can cause in your Outlook Error display like zzz.pst is not a personal folders file As one or more parameters are not valid so can’t move items or could not complete operation.