It appears the installation guide for APM server is introducing a repository URL with older packages. I checked the installation guide for Kibana and the URL for the APT repo was different (https://artifacts.elastic.co/packages/6.x/apt) and it hints what URL to use to get the latest versions of APM server, instead of the URL APM server guide is using (https://artifacts.elastic.co/packages/6.x-prerelease/apt).