First you need to check that you have enough space on your server for the backup , then do the following:
1.Go to https://<server_ip_address>:8443 and log into your Plesk control panel
2.Go to Home > Server > Backup Manager> Personal FTP Repository settings
3.Fill in the form fields and click OK
4.Go back to Backup Manager and then click on Back Up and select the Personal FTP repository radio button.