Comment 4 for bug 1833566

The same happened to me, and it was about secureboot. I had to import the livepatch key, like explained here: https://wiki.ubuntu.com/Kernel/Livepatch

Related bug: https://bugs.launchpad.net/ubuntu/+source/update-notifier/+bug/1833277

There, it's about the notification telling you what to do, but I think the snap utility should do the same, instead of just saying "apply-failed"