Core18 snap fails to run on 16.04 - cannot locate the core snap

snapd
3

$ snap version

snap    2.34.2ubuntu0.1
snapd   2.34.2ubuntu0.1
series  16
ubuntu  16.04
kernel  4.15.0-46-generic

$ snap list

Name              Version  Rev  Tracking  Publisher  Notes
andy-testsnap-py  0.5.2    5    edge      andyxxxxx  -
core18            18       731  stable    canonical  base