Nextcloud is not installed in LXC container, how to win?

Question

linux

Hello.
Once again I can't install nextcloud on linux. It runs from proxmox image Ubuntu 20*
here is the error code

[email protected]:~# sudo snap install nextcloud
error: system does not fully support snapd: cannot mount squashfs image using "squashfs": mount:
       /tmp/sanity-mountpoint-565358445: mount failed: Operation not permitted.

The funny thing is that the same nextcloud is installed without problems from the Turnkey linux assembly - but this is some kind of shnyag. Very crooked and peculiar distr. It is not clear what is inside and how it works.
How to win

error: system does not fully support snapd: cannot mount squashfs image using "squashfs": mount:
       /tmp/sanity-mountpoint-565358445: mount failed: Operation not permitted

. ???

Threat I searched on the internet - but whether I was looking for something or the recipes didn’t fit, what does he even need?
Thanks