I have the hello snap, notes and core installed on my computer and all of them work fine.
$ snap list
Name Version Rev Developer Notes
core 16.04.1 714 canonical -
hello 2.10 20 canonical -
notes 0.9.0~gitb6e3b34 4 notes-developers -
When the hello snap is installed, the core snap is installed along with it if core is not already installed. Try updating the core snap:
sudo snap refresh core
If core is already installed the following message will be shown:
snap "core" has no updates available
Your results of running
sudo snap refresh core were as follows:
sudo snap refresh core error: cannot refresh "core": cannot find snap "core"
So uninstall the core snap and then reinstall it.
sudo snap remove core hello
sudo snap install hello # core will also be installed automatically
Hello is a terminal app that runs like this.