Hi - I had a problem with this on Ubuntu 18.04.1 LTS (bionic). After installing the packages postfix and sasl2-bin, I then (after at least an hour of debugging), ended up putting in a symlink from /var/run/saslauthd to /var/spool/postfix/var/run/saslauthd.
I didn't change much from the standard configuration - just added /etc/postfix/sasl/smtpd.conf and uncommented the stuff in master.cf to listen on port 587. In particular, I didn't alter the chroot setting - it was chroot out of the box. I think something needs to be added to, perhaps, /usr/lib/postfix/configure-instance.sh, to create a bind mount for this directory when postfix starts. Maybe that's not quite right, since there's a sequencing issue for the saslauthd/postfix startup order, but something like this is in order. It should work out of the box, without having to move around system directories or manually add bind mounts. agape brent