From server console:

$> nano /etc/pve/lxc/{machine id, ex:100}.conf

add: 

lxc.cgroup2.devices.allow: c 10:200 rwm
lxc.mount.entry: /dev/net dev/net none bind,create=dir

$> chown 100000:100000 /dev/net/tun
$> chmod 666 /dev/net/tun

$> ls -l /dev/net/tun

Restart machine