It fails because the Puppetboard module makes faulty assumptions in its params.pp:
case $facts['os']['family'] {
'Debian': {
if ($facts['os']['name'] == 'ubuntu') {
$apache_confd = '/etc/apache2/conf-enabled'
} else {
$apache_confd = '/etc/apache2/conf.d'
}
$apache_service = 'apache2'
}
In Debian 9 the correct directory is /etc/apache2/conf-enabled.