Is it possible to install MicroK8s on Ubuntu core 18?

Ubuntu Core only supports strictly confined snaps.

Ubuntu Core does not support --classic snaps. That is why you are seeing the "classic" error on a Core system, but not on a Cloud system.

Here is an excerpt from the relevant release announcement (January 2017):

With snapd 2.20, a new confinement policy is introduced: “classic”, designed to cater for all your scripting and tooling needs. Snaps declaring their confinement as “classic”, have access to the rest of the system, as most legacy (debian packages for example) packaged apps do, while still benefiting from the ci-integrated store model, with automated updates, rollbacks to older versions, release channels, etc.

This new “classic” confinement is only available when snapd is installed on top of a traditional Linux distribution, as opposed to Ubuntu Core systems.

We are actively working on getting a strictly confined snap to allow MicroK8s to run on Ubuntu Core. There is a branch where you could follow the progress https://github.com/ubuntu/microk8s/tree/feature/jdb%2Fstrict . For now, the builds go in the latest/edge/strict channel. You can try this with:

sudo snap install microk8s --channel=latest/edge/strict

In case something does not work as you'd wish, we would appreciate your feedback. A simple inspection report should be enough (microk8s inspect). Thank you.