Might be that our documentation isnt the best here, but below is a full iteration of building/developing a snap on a core18 device … (for convenience you might want to manually add your user to the lxd group in /var/lib/extrausers/group, that saves you from needing sudo with the lxc commands)
$ grep Model /proc/cpuinfo
Model : Raspberry Pi 3 Model B Plus Rev 1.3
$ grep PRETTY /etc/os-release
PRETTY_NAME="Ubuntu Core 18"
$ snap install lxd
$ sudo lxd init --auto
$ sudo lxc launch ubuntu:18.04 bionic
Creating bionic
Starting bionic
$ sudo lxc shell bionic
root@bionic:~# snap install snapcraft --classic
...
root@bionic:~# git clone https://github.com/ogra1/htpdate-daemon-snap.git
root@bionic:~# cd htpdate-daemon-snap/
root@bionic:~/htpdate-daemon-snap# snapcraft --destructive-mode
...
Snapped 'htpdate-daemon_1.2.2_armhf.snap'
root@bionic:~/htpdate-daemon-snap# exit
logout
$ sudo lxc file pull bionic/root/htpdate-daemon-snap/htpdate-daemon_1.2.2_armhf.snap .
$ snap install --dangerous htpdate-daemon_1.2.2_armhf.snap
htpdate-daemon 1.2.2 installed
$