I’m running Bionic for the host, but I’ve also seen this in Travis (which is Trusty), using the lxd snap from 3.0/stable. Let me show you:
$ lxc launch ubuntu:bionic -e
Creating the container
Container name is: organic-penguin
$ lxc exec organic-penguin -- snap install hello
2018-09-12T20:59:10Z INFO Waiting for restart...
hello 2.10 from 'canonical' installed
$ lxc exec organic-penguin -- hello
cannot perform operation: mount --rbind /lib/modules /tmp/snap.rootfs_8u8wnr//lib/modules: No such file or directory
All snaps fail in this way. Do we still not have LXD tests in snapd? There’s spread coverage of so many distros, would like to see LXD xenial and bionic in there.