Why the "Back up" button is not avaliable under Backup Manager?
Backup service node was not added to Plesk Automation. If there are no backup nodes registered in PA, the backup and restore functions cannot be used, and the corresponding buttons are not available to users in the Customer Control Panel.
Add Backup service node using instructions from the documentation.
Open required service template and make sure that Data backup and restoration using the server repository option is activated:
Top > Products > Service Templates > ..service_template_name.. > Parameters