backup exchange 2013 error

Please go through NetBackup for Microsoft Exchange Server Administrator's Guide  for Policy config (download the pdf from the Attachments link).

You cannot backup mailboxes with Exch 2013.
Extract from above manual:

Mailbox and public folder operations are performed with MAPI. These operations
require the use of the Microsoft Exchange Mailboxes:\ or the Microsoft
Exchange Public Folders:\ directives in the backup policy. This type of backup
is only available with Exchange 2007. This type of restore is only available with
Exchange 2007 and 2003.
Symantec recommends that you perform streaming or snapshot backups. MAPI
mailbox and public folder backups allow for mailbox or public folder recovery, but
are not adequate for disaster recovery. Exchange database backups are required
for disaster recovery. Backups with Granular Recovery Technology (GRT) enabled
can replace MAPI mailbox and public folder backups.

As per the DB SCL, GRT support for Exch began in 7.6.0.3.

Herewith detailed instructions to config GRT backups:

Everything you need to know to get Exchange Granular backups to work:
http://www.symantec.com/docs/HOWTO73076  

Disk Storage Types supported for Granular Recovery Technology (GRT)
http://www.symantec.com/docs/TECH187917