cannot open snap apps - "cannot perform operation: mount --rbind /dev /tmp/snap.rootfs_9Cf5Oo//dev: no such file or directory"

I struggled with this same problem for a long time, and finally managed to solve it. The problem was because of my own setup, so it might not be applicable in you case, but maybe it'll point you in the right direction.

The problem seems to be that snap doesn't like having different things symlinked to different devices.

In my case, the problem was that my /var partition really was a symlink to /mnt/var. I had the correct mount point /var set in /etc/fstab, but of course fstab followed the symlink as well, and mounted the device to /mnt.

For me, the solution was to just rename the symlink with sudo mv /var /var.BAK, create an empty mount point with sudo mkdir /var and reboot. /etc/fstab now mounts the device directly to /var, and snap works like a charm.

So, try and do ls -l / and check that var is a directory and not a symlink (which would read for instance var -> /mnt/var). Also check that other directories snap might use (/tmp, /var/tmp, /var/snap, for instance) are real directories and not symlinks.