Auto-renewal is only available for memberships which expire on a rolling anniversary date, not for hard expiration dates where everyone expires on the same day (for example 12/31.) If you enable auto-renewal, payments must be made with a credit card and cannot be set to pay-later. Recurring payments can also be set up for auto-renewal.  It is recommended that you make it clear in the description area and thank you message that the membership will be automatically renewed.