There is a warning or error symbol next to my snapshot sizes, when I hover over it displays the message “Error: libvirt/images/pool-1: permission denied.” I’m on debian 12 backing up a locally mounted media archive to an smb mount.
I think the error message is self-explanatory. This is what happens often when you try to backup live system.
My suggestion would be to always utilise filesystem snapshots as explained here - Actions | Kopia.
It is only possible for ZFS/BTRFS/LVM. If not an option for your system then make sure that your backup contains only folders/files accessible by kopia user - it includes both permissions and guarantee that they are not used/locked by other programs during backup.