WARN: could not reopen tty: Permission denied

I’m trying to launch an unprivileged debian-bullseye-amd64 (the download template) container on Debian 8. The issue I’m having right now is the warning in the title e.g. when attaching to a container. I saw a possible solution in the other thread, but either I don’t know how to apply it, or it doesn’t work:

# mount
devpts on /dev/pts type devpts (rw,relatime,gid=886437,mode=620,ptmxmode=666)

# mount -o rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 devpts /dev/pts
mount: /dev/pts: permission denied.

If you could not only give some command, but also explain why it should be that way and not the other, that would be awesome.

It appears I was trying to attach from root:

# su - u1 -c 'lxc-attach -n c1'

Which supposedly explains the message.