Please add these lines to /etc/apt/sources.list or create the file /etc/apt/sources.list.d/lxqt-debian-net.list containing these lines:
deb https://lxqt.debian.net/debian experimental-snapshots main
deb-src https://lxqt.debian.net/debian experimental-snapshots main
apt-key adv --keyserver pgpkeys.mit.edu --recv-key 0x3A05D5979A01C951