This error message means that there is no archive(zip) utility installed for your site. This utility is used to make backups and also to zip up the log files that you will send to us, to resolve this you will need to talk to your hosting company they will need to install a php zip utility for you to use WPBackItUp. Most hosts support this and will install it for you.
Below is a link you can send them for information.
Note: If you need further assistance please get in touch with WPBackItUp Support and submit a ticket.