Ubuntu – System does not fully support snapd: cannot mount squashfs image using “squashfs – mount failed

I'm attempting to install PowerShell on my Ubuntu instance using snap, but it generates an error:

# sudo snap install powershell --classic
error: system does not fully support snapd: cannot mount squashfs image using "squashfs": mount:
       /tmp/sanity-mountpoint-072786794: mount failed: Operation not permitted.

I'm running Ubuntu Eoan in a LXC container on a Turris Omnia router:

# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 19.10
Release:    19.10
Codename:   eoan

# uname -m
armv7l

# snap --version
snap    2.42.1+19.10
snapd   2.42.1+19.10
series  16
ubuntu  19.10
kernel  4.14.162

I'm able to create a directory in /tmp.

This posting suggests that I need at least kernel 4.4.0-6.21; I have 4.14.162.

Why is this failing?