Did it work before? Did anything change on the system? Do you start the backup manually or via Backup Schedule? Does the backup actually run (do you see the file on the target-storage)? please provide a bit more information, so that we have a chance to help you.
Please post the output of pveversion -v, and the contents of /etc/pve/vzdump.cron (if it's a scheduled backup), and also make sure that the symlink of /etc/cron.d/vzdump -> /etc/pve/vzdump.cron is there