Cannot install Puppet 5.5 on Ubuntu Server 18.04.1 LTS

Puppet Version: 5.5
Puppet Server Version: 5.3
OS Name/Version: Ubuntu Server 18.04 LTS

I cannot install the puppetserver package on my clean Ubuntu Server 18.04.1 VM because it depends on openjdk-8-jre-headless which isn't available.

I followed the following instructions from here for adding the repositories to my newly installed Ubuntu 18.04.1 server.

wget https://apt.puppetlabs.com/puppet5-release-bionic.deb

sudo dpkg -i puppet5-release-bionic.deb

sudo apt update

Which adds a sources file with this in it:

1. Puppet 5 bionic Repository
   deb http://apt.puppetlabs.com bionic puppet5

1. Puppet 5 bionic Source Repository
2. The source repos are commented out by default because we
3. do not always make sources available for all packages or
4. for all platforms. If you want to access the source repos,
5. uncomment the following line.
   #deb-src http://apt.puppetlabs.com bionic puppet5

...looks fine.

The apt-get update then gives this:

Ign:1 http://apt.puppetlabs.com bionic InRelease
Get:2 http://apt.puppetlabs.com bionic Release [40.4 kB]
Hit:3 http://archive.ubuntu.com/ubuntu bionic InRelease
{{Get:4 http://apt.puppetlabs.com bionic Release.gpg [819 B] }}
Get:5 http://archive.ubuntu.com/ubuntu bionic-security InRelease [83.2 kB]
Get:6 http://apt.puppetlabs.com bionic/puppet5 all Packages [1,676 B]
{{Get:7 http://apt.puppetlabs.com bionic/puppet5 amd64 Packages [6,007 B] }}
Get:8 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]

...looking ok, it got package lists from you.

However, when I run sudo apt-get install puppetserver I get this message:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
puppetserver : Depends: openjdk-8-jre-headless but it is not installable
E: Unable to correct problems, you have held broken packages.

The current version is openjdk-11-jre-headless, I tried installing that but, unsurprisingly, it didn't help.

I can't see anything in your instructions regarding openjdk-8-jre-headless, I assume this should work, or else you wouldn't just have the code to copy for installing on Ubuntu 18.04. Maybe I'm doing something wrong, but I'm fairly certain I followed the instructions correctly.

Desired Behavior:

It installs puppetserver

Actual Behavior:

It doesn't install puppetserver

I don't think I have any logs yet, I haven't managed to install it.