Bionic has been EOL for a few months now, and building PostgreSQL 16
on bionic turned out to be difficult (among other problems, zstd being
available but too old), so I used that to stop supporting bionic on
apt.postgresql.org. No new builds for that distributions have been
made since July.
bionic-pgdg has now been copied to apt-archive.postgresql.org and will
be removed from apt.postgresql.org at the end of October 2023.
If you are still on bionic (please upgrade!) point your sources.list
entries to https://apt-archive.postgresql.org.
deb https://apt-archive.postgresql.org/pub/repos/apt bionic-pgdg main
Christoph