PuppetDB 7.0 dropped support for PostgresSQL versions older than 11.0:
Running PuppetDB with PostgreSQL 9.6 or 10 is no longer supported. Use PostgreSQL 11 or greater instead.
https://puppet.com/docs/puppetdb/latest/release_notes.html#upgrading
However, version 7.7.1 of the puppetlabs-puppetdb module still defaults to installing PostgreSQL 9.6:
https://github.com/puppetlabs/puppetlabs-puppetdb/blob/7.7.1/manifests/params.pp#L27