Sharepoint library but not list synching to Outlook 365

Hi all

I am wanting to sync a Sharepoint list with Outlook 365 contacts.  I can sync a library in sharepoint (which doesn't show subfolders) in Outlook. But am unable to sync a list in Sharepoint to Outlook as the sync icon is greyed out.  I have synced a list before, so don't understand why now I can only sync a library.

Is there a way for the Outlook (under sharepoint lists) to show the subfolders created?

Your help is appreciated