Resource mailboxes configured for direct booking aren’t automatically accepting meetings
Direct Booking has been deprecated in Outlook 2013 – Changes in Office 2013
Convert the (resource) mailbox from user to a room mailbox.
How to Convert a Mailbox ?
How to Enable or Disable Automatic Booking on a Resource Mailbox
How to Upgrade Outlook Direct Booking Resource Mailboxes to Exchange 2007
Thanks & Regards