Some distribution packages of snapd change the /snap directory to be /var/lib/snapd/snap. This is fine for most snaps, however it is a problem for snaps with classic confinement as they get an unaltered view on the host filesystems and a non-default location will break some of them.
When https://github.com/snapcore/snapd/pull/3693 lands we will support that users on such distros can fix the situation themself by creating a symlink from /var/lib/snapd/snap to /snap.
The current proposal for the message on non /snap distributions is:
“your distribution choose a directory layout incompatible with classic confinement. You can fix this yourself by setting a symlink from $SnapMountDir to /snap”
Feedback/suggestions on this wording welcome!