root@websyncbox:~# cat apt_postgresql_org_pub_repos_apt.list
deb http://apt.postgresql.org/pub/repos/apt/ bionic-pgdg main

root@websyncbox:~# apt-get update
Hit:1 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://security.ubuntu.com/ubuntu bionic-security InRelease       
Hit:3 http://archive.ubuntu.com/ubuntu bionic-updates InRelease         
Hit:4 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Reading package lists... Done                      
root@websyncbox:~# 
root@websyncbox:~# 
root@websyncbox:~# apt-get install postgresql-9.1
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package postgresql-9.1 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'postgresql-9.1' has no installation candidate