/etc/apt/sources.list.d/debian.list on latest lxqt .iso has
deb https://deb.debian.org/debian/ unstable main contrib non-free-firmware
that is, non-free is missing.
I think this error is also there on other .iso and on older ones, probably back to when non-free-firmware was introduced.
We need non-free for codecs, etc. -- everyone should check their sources.list to make sure it includes non-free
It in the XFCE4 versions sources.list