Quick info: you cannot backup the Host server with PBS, only VMs and LXCs.

Edit: there seems to be some ways to send a manual backup of the PVE host to PBS but it seems to be CLI only and I don't think the GUI allows that.

Also, you might want to put the backups in a dedicated partition or drive if you can, so you'd just have to reinstall pbs in case it get bad. I don't know any way to backup pbs, as I don't do that myself, maybe backuppc ?