The inability to install UniFi OS Server on a LXC container is caused solely by the UOS installer implementation.
The installer aborts while attempting to set sysctl parameters that are not absolutely necessary for its operation.
To ignore those errors, you can create a wrapper at e.g. /usr/local/sbin/sysctl (it has higher priority in $PATH than sysctl binary) what will forward everything to the original sysctl, but ignore potential errors:
#!/bin/sh
/usr/sbin/sysctl $@ || true
exit 0
Then
chmod +x /usr/local/sbin/sysctl
then reboot or reconnect to the shell.
Additionally, you need to grant the LXC container the appropriate permissions so that the Podman can create the tun interface.
Method 1) Go to the Container Resources > add > Device Passthrough:
Advanced checked
Device path: /dev/net/tun
Access mode in CT: 0666
Method 2) Edit /etc/pve/lxc/CT_ID.conf and add:
lxc.cgroup2.devices.allow: c 10:200 rwm
lxc.mount.entry: /dev/net/tun dev/net/tun none bind,create=file
This way, you can successfully run UniFi OS Server installer on the LXC.
This instruction only work for privileged LXC container, it won't work for unprivileged container.